diff --git a/toolkit/components/osfile/.eslintrc.js b/toolkit/components/osfile/.eslintrc.js
new file mode 100644
index 000000000000..60fee928a673
--- /dev/null
+++ b/toolkit/components/osfile/.eslintrc.js
@@ -0,0 +1,14 @@
+"use strict";
+
+module.exports = {
+  "rules": {
+    "consistent-return": "off",
+    "no-irregular-whitespace": "off",
+    "no-lone-blocks": "off",
+    "no-redeclare": "off",
+    "no-self-assign": "off",
+    "no-shadow": "off",
+    "no-unused-vars": "off",
+    "object-shorthand": "off",
+  }
+};
diff --git a/toolkit/components/osfile/modules/osfile_async_front.jsm b/toolkit/components/osfile/modules/osfile_async_front.jsm
index b92d00e48748..a7187dac2ecb 100644
--- a/toolkit/components/osfile/modules/osfile_async_front.jsm
+++ b/toolkit/components/osfile/modules/osfile_async_front.jsm
@@ -261,7 +261,7 @@ var Scheduler = this.Scheduler = {
   /**
    * Restart the OS.File worker killer timer.
    */
-  restartTimer(arg) {
+  restartTimer: function(arg) {
     this.hasRecentActivity = true;
   },
 
@@ -275,7 +275,7 @@ var Scheduler = this.Scheduler = {
    *   would not cause leaks. Otherwise, assume that the worker will be shutdown
    *   through some other mean.
    */
-  kill({shutdown, reset}) {
+  kill: function({shutdown, reset}) {
     // Grab the kill queue to make sure that we
     // cannot be interrupted by another call to `kill`.
     let killQueue = this._killQueue;
@@ -382,7 +382,7 @@ var Scheduler = this.Scheduler = {
    * @return {Promise} A promise with the same behavior as
    * the promise returned by |code|.
    */
-  push(code) {
+  push: function(code) {
     let promise = this.queue.then(code);
     // By definition, |this.queue| can never reject.
     this.queue = promise.catch(() => undefined);
@@ -482,7 +482,7 @@ var Scheduler = this.Scheduler = {
    *
    * This is only useful on first launch.
    */
-  _updateTelemetry() {
+  _updateTelemetry: function() {
     let worker = this.worker;
     let workerTimeStamps = worker.workerTimeStamps;
     if (!workerTimeStamps) {
@@ -1270,10 +1270,10 @@ var DirectoryIterator = function DirectoryIterator(path, options) {
   this._isClosed = false;
 };
 DirectoryIterator.prototype = {
-  iterator() {
+  iterator: function() {
     return this;
   },
-  __iterator__() {
+  __iterator__: function() {
     return this;
   },
 
@@ -1481,7 +1481,7 @@ this.OS.Path = Path;
 
 // Returns a resolved promise when all the queued operation have been completed.
 Object.defineProperty(OS.File, "queue", {
-  get() {
+  get: function() {
     return Scheduler.queue;
   }
 });
@@ -1500,7 +1500,7 @@ var Barriers = {
   /**
    * Return the shutdown state of OS.File
    */
-  getDetails() {
+  getDetails: function() {
     let result = {
       launched: Scheduler.launched,
       shutdown: Scheduler.shutdown,
diff --git a/toolkit/components/osfile/modules/osfile_async_worker.js b/toolkit/components/osfile/modules/osfile_async_worker.js
index ce25bae69776..c54ef426b605 100644
--- a/toolkit/components/osfile/modules/osfile_async_worker.js
+++ b/toolkit/components/osfile/modules/osfile_async_worker.js
@@ -59,7 +59,7 @@ if (this.Components) {
    /**
     * Get a resource from its unique identifier.
     */
-   get(id) {
+   get: function(id) {
      let result = this._map.get(id);
      if (result == null) {
        return result;
@@ -69,7 +69,7 @@ if (this.Components) {
    /**
     * Remove a resource from its unique identifier.
     */
-   remove(id) {
+   remove: function(id) {
      if (!this._map.has(id)) {
        throw new Error("Cannot find resource id " + id);
      }
@@ -84,9 +84,9 @@ if (this.Components) {
     * @return {*} A unique identifier. For the moment, this is a number,
     * but this might not remain the case forever.
     */
-   add(resource, info) {
+   add: function(resource, info) {
      let id = this._idgen++;
-     this._map.set(id, {resource, info});
+     this._map.set(id, {resource: resource, info: info});
      return id;
    },
    /**
@@ -154,12 +154,12 @@ if (this.Components) {
   */
   let Agent = {
    // Update worker's OS.Shared.DEBUG flag message from controller.
-   SET_DEBUG(aDEBUG) {
+   SET_DEBUG: function(aDEBUG) {
      SharedAll.Config.DEBUG = aDEBUG;
    },
    // Return worker's current OS.Shared.DEBUG value to controller.
    // Note: This is used for testing purposes.
-   GET_DEBUG() {
+   GET_DEBUG: function() {
      return SharedAll.Config.DEBUG;
    },
    /**
@@ -168,7 +168,7 @@ if (this.Components) {
     * @param {bool} If |true|, kill the worker if this would not cause
     * leaks.
     */
-   Meta_shutdown(kill) {
+   Meta_shutdown: function(kill) {
      let result = {
        openedFiles: OpenedFiles.listOpenedResources(),
        openedDirectoryIterators: OpenedDirectoryIterators.listOpenedResources(),
@@ -269,7 +269,7 @@ if (this.Components) {
                              options
                             );
    },
-   removeDir(path, options) {
+   removeDir: function(path, options) {
      return File.removeDir(Type.path.fromMsg(path), options);
    },
    new_DirectoryIterator: function new_DirectoryIterator(path, options) {
diff --git a/toolkit/components/osfile/modules/osfile_shared_allthreads.jsm b/toolkit/components/osfile/modules/osfile_shared_allthreads.jsm
index 63edd5512d0a..f5a8dd3e9ae9 100644
--- a/toolkit/components/osfile/modules/osfile_shared_allthreads.jsm
+++ b/toolkit/components/osfile/modules/osfile_shared_allthreads.jsm
@@ -109,7 +109,7 @@ var defineLazyGetter = function defineLazyGetter(object, name, getter) {
       delete this[name];
       let value = getter.call(this);
       Object.defineProperty(object, name, {
-        value
+        value: value
       });
       return value;
     }
@@ -213,10 +213,10 @@ var clone = function(object, refs = []) {
   let refer = function refer(result, key, object) {
     Object.defineProperty(result, key, {
       enumerable: true,
-      get() {
+      get: function() {
         return object[key];
       },
-      set(value) {
+      set: function(value) {
         object[key] = value;
       }
     });
@@ -301,7 +301,7 @@ Type.prototype = {
       this);
     Object.defineProperty(this, "in_ptr",
     {
-      get() {
+      get: function() {
         return ptr_t;
       }
     });
@@ -319,7 +319,7 @@ Type.prototype = {
       this);
     Object.defineProperty(this, "out_ptr",
     {
-      get() {
+      get: function() {
         return ptr_t;
       }
     });
@@ -341,7 +341,7 @@ Type.prototype = {
       this);
     Object.defineProperty(this, "inout_ptr",
     {
-      get() {
+      get: function() {
         return ptr_t;
       }
     });
@@ -865,7 +865,7 @@ HollowStructure.prototype = {
                       " at offset " + offset +
                       " without exceeding its size of " + this.size);
     }
-    let field = {name, type};
+    let field = {name: name, type: type};
     this.offset_to_field_info[offset] = field;
   },
 
@@ -991,7 +991,7 @@ Library.prototype = Object.freeze({
     }
     let error = new Error("Could not open library " + this.name);
     Object.defineProperty(this, "library", {
-      get() {
+      get: function() {
         throw error;
       }
     });
@@ -1010,10 +1010,10 @@ Library.prototype = Object.freeze({
    * @param {Type} returnType The type of values returned by the function.
    * @param {...Type} argTypes The type of arguments to the function.
    */
-  declareLazyFFI(object, field, ...args) {
+  declareLazyFFI: function(object, field, ...args) {
     let lib = this;
     Object.defineProperty(object, field, {
-      get() {
+      get: function() {
         delete this[field];
         let ffi = declareFFI(lib.library, ...args);
         if (ffi) {
@@ -1037,10 +1037,10 @@ Library.prototype = Object.freeze({
    * @param {ctypes.CType} returnType The type of values returned by the function.
    * @param {...ctypes.CType} argTypes The type of arguments to the function.
    */
-  declareLazy(object, field, ...args) {
+  declareLazy: function(object, field, ...args) {
     let lib = this;
     Object.defineProperty(object, field, {
-      get() {
+      get: function() {
         delete this[field];
         let ffi = lib.library.declare(...args);
         if (ffi) {
@@ -1067,10 +1067,10 @@ Library.prototype = Object.freeze({
    * @param {ctypes.CType} returnType The type of values returned by the function.
    * @param {...ctypes.CType} argTypes The type of arguments to the function.
    */
-  declareLazyWithFallback(fallbacklibrary, object, field, ...args) {
+  declareLazyWithFallback: function(fallbacklibrary, object, field, ...args) {
     let lib = this;
     Object.defineProperty(object, field, {
-      get() {
+      get: function() {
         delete this[field];
         try {
           let ffi = lib.library.declare(...args);
@@ -1089,7 +1089,7 @@ Library.prototype = Object.freeze({
     });
   },
 
-  toString() {
+  toString: function() {
     return "[Library " + this.name + "]";
   }
 });
@@ -1126,6 +1126,7 @@ var declareFFI = function declareFFI(lib, symbol, abi,
     throw new TypeError("declareFFI expects as third argument an instance of Type");
   }
   let signature = [symbol, abi];
+  let argtypes  = [];
   for (let i = 3; i < arguments.length; ++i) {
     let current = arguments[i];
     if (!current) {
@@ -1175,7 +1176,7 @@ exports.declareFFI = declareFFI;
  */
 function declareLazyFFI(object, field, ...declareFFIArgs) {
   Object.defineProperty(object, field, {
-    get() {
+    get: function() {
       delete this[field];
       let ffi = declareFFI(...declareFFIArgs);
       if (ffi) {
@@ -1203,7 +1204,7 @@ exports.declareLazyFFI = declareLazyFFI;
  */
 function declareLazy(object, field, lib, ...declareArgs) {
   Object.defineProperty(object, field, {
-    get() {
+    get: function() {
       delete this[field];
       try {
         let ffi = lib.declare(...declareArgs);
@@ -1278,31 +1279,31 @@ exports.OSError = OSError;
 exports.OS = {
   Constants: exports.Constants,
   Shared: {
-    LOG,
-    clone,
-    Type,
-    HollowStructure,
+    LOG: LOG,
+    clone: clone,
+    Type: Type,
+    HollowStructure: HollowStructure,
     Error: OSError,
-    declareFFI,
-    projectValue,
-    isTypedArray,
-    defineLazyGetter
+    declareFFI: declareFFI,
+    projectValue: projectValue,
+    isTypedArray: isTypedArray,
+    defineLazyGetter: defineLazyGetter
   }
 };
 
 Object.defineProperty(exports.OS.Shared, "DEBUG", {
-  get() {
+  get: function() {
     return Config.DEBUG;
   },
-  set(x) {
+  set: function(x) {
     return Config.DEBUG = x;
   }
 });
 Object.defineProperty(exports.OS.Shared, "TEST", {
-  get() {
+  get: function() {
     return Config.TEST;
   },
-  set(x) {
+  set: function(x) {
     return Config.TEST = x;
   }
 });
diff --git a/toolkit/components/osfile/modules/osfile_shared_front.jsm b/toolkit/components/osfile/modules/osfile_shared_front.jsm
index cbecbc902521..335e76f732d1 100644
--- a/toolkit/components/osfile/modules/osfile_shared_front.jsm
+++ b/toolkit/components/osfile/modules/osfile_shared_front.jsm
@@ -22,7 +22,7 @@ var SharedAll =
 var Path = require("resource://gre/modules/osfile/ospath.jsm");
 var Lz4 =
   require("resource://gre/modules/lz4.js");
-SharedAll.LOG.bind(SharedAll, "Shared front-end");
+var LOG = SharedAll.LOG.bind(SharedAll, "Shared front-end");
 var clone = SharedAll.clone;
 
 /**
@@ -152,7 +152,7 @@ AbstractFile.openUnique = function openUnique(path, options = {}) {
 
   try {
     return {
-      path,
+      path: path,
       file: OS.File.open(path, mode)
     };
   } catch (ex) {
@@ -179,7 +179,6 @@ AbstractFile.openUnique = function openUnique(path, options = {}) {
       }
       throw OS.File.Error.exists("could not find an unused file name.", path);
     }
-    throw ex;
   }
 };
 
diff --git a/toolkit/components/osfile/modules/osfile_unix_allthreads.jsm b/toolkit/components/osfile/modules/osfile_unix_allthreads.jsm
index e3e8b34bf9d5..5685b315cf40 100644
--- a/toolkit/components/osfile/modules/osfile_unix_allthreads.jsm
+++ b/toolkit/components/osfile/modules/osfile_unix_allthreads.jsm
@@ -37,7 +37,7 @@ if (typeof Components != "undefined") {
   throw new Error("Please open this module with Component.utils.import or with require()");
 }
 
-SharedAll.LOG.bind(SharedAll, "Unix", "allthreads");
+var LOG = SharedAll.LOG.bind(SharedAll, "Unix", "allthreads");
 var Const = SharedAll.Constants.libc;
 
 // Open libc
diff --git a/toolkit/components/osfile/modules/osfile_unix_back.jsm b/toolkit/components/osfile/modules/osfile_unix_back.jsm
index 03c2e571b7fe..e4e076043ca1 100644
--- a/toolkit/components/osfile/modules/osfile_unix_back.jsm
+++ b/toolkit/components/osfile/modules/osfile_unix_back.jsm
@@ -5,7 +5,6 @@
 /* eslint-env mozilla/chrome-worker, node */
 /* global OS */
 
-// eslint-disable-next-line no-lone-blocks
 {
   if (typeof Components != "undefined") {
     // We do not wish osfile_unix_back.jsm to be used directly as a main thread
@@ -25,7 +24,7 @@
        require("resource://gre/modules/osfile/osfile_shared_allthreads.jsm");
      let SysAll =
        require("resource://gre/modules/osfile/osfile_unix_allthreads.jsm");
-     SharedAll.LOG.bind(SharedAll, "Unix", "back");
+     let LOG = SharedAll.LOG.bind(SharedAll, "Unix", "back");
      let libc = SysAll.libc;
      let Const = SharedAll.Constants.libc;
 
@@ -36,18 +35,19 @@
       */
      // FIXME: Both |init| and |aDeclareFFI| are deprecated, we should remove them
      let init = function init(aDeclareFFI) {
+       let declareFFI;
        if (aDeclareFFI) {
-         aDeclareFFI.bind(null, libc);
+         declareFFI = aDeclareFFI.bind(null, libc);
        } else {
-         SysAll.declareFFI;
+         declareFFI = SysAll.declareFFI;
        }
-       SharedAll.declareLazyFFI;
+       let declareLazyFFI = SharedAll.declareLazyFFI;
 
        // Initialize types that require additional OS-specific
        // support - either finalization or matching against
        // OS-specific constants.
        let Type = Object.create(SysAll.Type);
-       let SysFile = exports.OS.Unix.File = { Type };
+       let SysFile = exports.OS.Unix.File = { Type: Type };
 
        /**
         * A file descriptor.
diff --git a/toolkit/components/osfile/modules/osfile_unix_front.jsm b/toolkit/components/osfile/modules/osfile_unix_front.jsm
index f5eb697f30a3..f1dd8ebbc550 100644
--- a/toolkit/components/osfile/modules/osfile_unix_front.jsm
+++ b/toolkit/components/osfile/modules/osfile_unix_front.jsm
@@ -12,7 +12,6 @@
 /* eslint-env mozilla/chrome-worker, node */
 /* global OS */
 
-// eslint-disable-next-line no-lone-blocks
 {
   if (typeof Components != "undefined") {
     // We do not wish osfile_unix_front.jsm to be used directly as a main thread
@@ -32,7 +31,7 @@
      let Path = require("resource://gre/modules/osfile/ospath.jsm");
      let SysAll = require("resource://gre/modules/osfile/osfile_unix_allthreads.jsm");
      exports.OS.Unix.File._init();
-     SharedAll.LOG.bind(SharedAll, "Unix front-end");
+     let LOG = SharedAll.LOG.bind(SharedAll, "Unix front-end");
      let Const = SharedAll.Constants.libc;
      let UnixFile = exports.OS.Unix.File;
      let Type = UnixFile.Type;
@@ -223,7 +222,7 @@
       */
      if (SharedAll.Constants.Sys.Name != "Android") {
        File.prototype.setDates = function(accessDate, modificationDate) {
-         let { /* value, */ ptr} = datesToTimevals(accessDate, modificationDate);
+         let {value, ptr} = datesToTimevals(accessDate, modificationDate);
          throw_on_negative("setDates",
            UnixFile.futimes(this.fd, ptr),
            this._path);
@@ -556,6 +555,7 @@
          // Perform actual copy
          let total_read = 0;
          while (true) {
+           let chunk_size = Math.min(nbytes, bufSize);
            let bytes_just_read = read(pump_buffer, bufSize);
            if (bytes_just_read == 0) {
              return total_read;
@@ -646,6 +646,7 @@
        // copy directories
        File.copy = function copy(sourcePath, destPath, options = {}) {
          let source, dest;
+         let result;
          try {
            source = File.open(sourcePath);
            // Need to open the output file with |append:false|, or else |splice|
@@ -656,9 +657,9 @@
              dest = File.open(destPath, {trunc: true, append: false});
            }
            if (options.unixUserland) {
-             pump_userland(source, dest, options);
+             result = pump_userland(source, dest, options);
            } else {
-             pump(source, dest, options);
+             result = pump(source, dest, options);
            }
          } catch (x) {
            if (dest) {
@@ -1001,7 +1002,7 @@
       * @throws {OS.File.Error} In case of I/O error.
       */
      File.setDates = function setDates(path, accessDate, modificationDate) {
-       let {/* value, */ ptr} = datesToTimevals(accessDate, modificationDate);
+       let {value, ptr} = datesToTimevals(accessDate, modificationDate);
        throw_on_negative("setDates",
                          UnixFile.utimes(path, ptr),
                          path);
@@ -1081,10 +1082,10 @@
       * Get/set the current directory.
       */
      Object.defineProperty(File, "curDir", {
-         set(path) {
+         set: function(path) {
            this.setCurrentDirectory(path);
          },
-         get() {
+         get: function() {
            return this.getCurrentDirectory();
          }
        }
diff --git a/toolkit/components/osfile/modules/osfile_win_allthreads.jsm b/toolkit/components/osfile/modules/osfile_win_allthreads.jsm
index 349de6589d21..3523f9fad141 100644
--- a/toolkit/components/osfile/modules/osfile_win_allthreads.jsm
+++ b/toolkit/components/osfile/modules/osfile_win_allthreads.jsm
@@ -37,7 +37,7 @@ if (typeof Components != "undefined") {
   throw new Error("Please open this module with Component.utils.import or with require()");
 }
 
-SharedAll.LOG.bind(SharedAll, "Win", "allthreads");
+var LOG = SharedAll.LOG.bind(SharedAll, "Win", "allthreads");
 var Const = SharedAll.Constants.Win;
 
 // Open libc
@@ -85,6 +85,7 @@ libc.declareLazy(Scope, "FormatMessage",
  */
 var OSError = function OSError(operation = "unknown operation",
                                lastError = ctypes.winLastError, path = "") {
+  operation = operation;
   SharedAll.OSError.call(this, operation, path);
   this.winLastError = lastError;
 };
diff --git a/toolkit/components/osfile/modules/osfile_win_back.jsm b/toolkit/components/osfile/modules/osfile_win_back.jsm
index ae3b9b48a52c..95c012634253 100644
--- a/toolkit/components/osfile/modules/osfile_win_back.jsm
+++ b/toolkit/components/osfile/modules/osfile_win_back.jsm
@@ -22,7 +22,6 @@
 /* eslint-env mozilla/chrome-worker, node */
 /* global OS */
 
-// eslint-disable-next-line no-lone-blocks
 {
   if (typeof Components != "undefined") {
     // We do not wish osfile_win.jsm to be used directly as a main thread
@@ -41,7 +40,7 @@
 
      let SharedAll = require("resource://gre/modules/osfile/osfile_shared_allthreads.jsm");
      let SysAll = require("resource://gre/modules/osfile/osfile_win_allthreads.jsm");
-     SharedAll.LOG.bind(SharedAll, "Unix", "back");
+     let LOG = SharedAll.LOG.bind(SharedAll, "Unix", "back");
      let libc = SysAll.libc;
      let advapi32 = new SharedAll.Library("advapi32", "advapi32.dll");
      let Const = SharedAll.Constants.Win;
@@ -53,19 +52,19 @@
       */
      // FIXME: Both |init| and |aDeclareFFI| are deprecated, we should remove them
      let init = function init(aDeclareFFI) {
-       let declareFFI; // eslint-disable-line no-unused-vars
+       let declareFFI;
        if (aDeclareFFI) {
          declareFFI = aDeclareFFI.bind(null, libc);
        } else {
          declareFFI = SysAll.declareFFI;
        }
-       let declareLazyFFI = SharedAll.declareLazyFFI; // eslint-disable-line no-unused-vars
+       let declareLazyFFI = SharedAll.declareLazyFFI;
 
        // Initialize types that require additional OS-specific
        // support - either finalization or matching against
        // OS-specific constants.
        let Type = Object.create(SysAll.Type);
-       let SysFile = exports.OS.Win.File = { Type };
+       let SysFile = exports.OS.Win.File = { Type: Type };
 
        // Initialize types
 
diff --git a/toolkit/components/osfile/modules/osfile_win_front.jsm b/toolkit/components/osfile/modules/osfile_win_front.jsm
index af5efd86c5d6..2aa463f3d33e 100644
--- a/toolkit/components/osfile/modules/osfile_win_front.jsm
+++ b/toolkit/components/osfile/modules/osfile_win_front.jsm
@@ -12,7 +12,6 @@
 /* eslint-env mozilla/chrome-worker, node */
 /* global OS */
 
-// eslint-disable-next-line no-lone-blocks
 {
   if (typeof Components != "undefined") {
     // We do not wish osfile_win_front.jsm to be used directly as a main thread
@@ -1154,10 +1153,10 @@
       * Get/set the current directory by |curDir|.
       */
      Object.defineProperty(File, "curDir", {
-         set(path) {
+         set: function(path) {
            this.setCurrentDirectory(path);
          },
-         get() {
+         get: function() {
            return this.getCurrentDirectory();
          }
        }
@@ -1196,6 +1195,40 @@
        return result;
      }
 
+     /**
+      * Utility function to sort errors represented as "-1" from successes.
+      *
+      * @param {string=} operation The name of the operation. If unspecified,
+      * the name of the caller function.
+      * @param {number} result The result of the operation that may
+      * represent either an error or a success. If -1, this function raises
+      * an error holding ctypes.winLastError, otherwise it returns |result|.
+      * @param {string=} path The path of the file.
+      */
+     function throw_on_negative(operation, result, path) {
+       if (result < 0) {
+         throw new File.Error(operation, ctypes.winLastError, path);
+       }
+       return result;
+     }
+
+     /**
+      * Utility function to sort errors represented as |null| from successes.
+      *
+      * @param {string=} operation The name of the operation. If unspecified,
+      * the name of the caller function.
+      * @param {pointer} result The result of the operation that may
+      * represent either an error or a success. If |null|, this function raises
+      * an error holding ctypes.winLastError, otherwise it returns |result|.
+      * @param {string=} path The path of the file.
+      */
+     function throw_on_null(operation, result, path) {
+       if (result == null || (result.isNull && result.isNull())) {
+         throw new File.Error(operation, ctypes.winLastError, path);
+       }
+       return result;
+     }
+
      /**
       * Helper used by both versions of setPermissions
       */
diff --git a/toolkit/components/osfile/tests/mochi/main_test_osfile_async.js b/toolkit/components/osfile/tests/mochi/main_test_osfile_async.js
index a4bbab83c73b..ff8b14371deb 100644
--- a/toolkit/components/osfile/tests/mochi/main_test_osfile_async.js
+++ b/toolkit/components/osfile/tests/mochi/main_test_osfile_async.js
@@ -407,7 +407,7 @@ var test_debug_test = maketest("debug_test", function debug_test(test) {
   return (async function() {
     // Create a console listener.
     let consoleListener = {
-      observe(aMessage) {
+      observe: function(aMessage) {
         // Ignore unexpected messages.
         if (!(aMessage instanceof Components.interfaces.nsIConsoleMessage)) {
           return;
@@ -420,7 +420,7 @@ var test_debug_test = maketest("debug_test", function debug_test(test) {
     };
     toggleDebugTest(true, consoleListener);
     // Execution of OS.File.exist method will trigger OS.File.LOG several times.
-    await OS.File.exists(EXISTING_FILE);
+    let fileExists = await OS.File.exists(EXISTING_FILE);
     toggleDebugTest(false, consoleListener);
   })();
 });
diff --git a/toolkit/components/osfile/tests/mochi/worker_test_osfile_front.js b/toolkit/components/osfile/tests/mochi/worker_test_osfile_front.js
index 66286b5eaaf0..765b163f059c 100644
--- a/toolkit/components/osfile/tests/mochi/worker_test_osfile_front.js
+++ b/toolkit/components/osfile/tests/mochi/worker_test_osfile_front.js
@@ -65,7 +65,7 @@ function test_open_non_existing_file() {
   info("Starting test_open_non_existing");
   let exn;
   try {
-    OS.File.open("/I do not exist");
+    let file = OS.File.open("/I do not exist");
   } catch (x) {
     exn = x;
     info("test_open_non_existing_file: Exception detail " + exn);
diff --git a/toolkit/components/osfile/tests/mochi/worker_test_osfile_shared.js b/toolkit/components/osfile/tests/mochi/worker_test_osfile_shared.js
index 5f847e2cb9f0..da82d4b0ab91 100644
--- a/toolkit/components/osfile/tests/mochi/worker_test_osfile_shared.js
+++ b/toolkit/components/osfile/tests/mochi/worker_test_osfile_shared.js
@@ -19,12 +19,12 @@ function ok(condition, description) {
 
 function is(a, b, description) {
   let outcome = a == b; // Need to decide outcome here, as not everything can be serialized
-  send({kind: "is", outcome, description: "" + description, a: "" + a, b: "" + b});
+  send({kind: "is", outcome: outcome, description: "" + description, a: "" + a, b: "" + b});
 }
 
 function isnot(a, b, description) {
   let outcome = a != b; // Need to decide outcome here, as not everything can be serialized
-  send({kind: "isnot", outcome, description: "" + description, a: "" + a, b: "" + b});
+  send({kind: "isnot", outcome: outcome, description: "" + description, a: "" + a, b: "" + b});
 }
 
 function info(description) {
diff --git a/toolkit/components/osfile/tests/mochi/worker_test_osfile_unix.js b/toolkit/components/osfile/tests/mochi/worker_test_osfile_unix.js
index 3aad24e05fd0..811f4210c919 100644
--- a/toolkit/components/osfile/tests/mochi/worker_test_osfile_unix.js
+++ b/toolkit/components/osfile/tests/mochi/worker_test_osfile_unix.js
@@ -118,7 +118,7 @@ function test_read_write() {
       break;
     }
     while (bytes > 0) {
-      array.addressOfElement(write_from);
+      let ptr = array.addressOfElement(write_from);
       // Note: |write| launches an exception in case of error
       let written = OS.Unix.File.write(output, array, bytes);
       isnot(written, -1, "test_read_write: no write error");
@@ -186,10 +186,10 @@ function test_passing_undefined() {
   let exceptionRaised = false;
 
   try {
-    OS.Unix.File.open(undefined, OS.Constants.libc.O_RDWR
-                                 | OS.Constants.libc.O_CREAT
-                                 | OS.Constants.libc.O_TRUNC,
-                                   OS.Constants.libc.S_IRWXU);
+    let file = OS.Unix.File.open(undefined, OS.Constants.libc.O_RDWR
+                                            | OS.Constants.libc.O_CREAT
+                                            | OS.Constants.libc.O_TRUNC,
+                                            OS.Constants.libc.S_IRWXU);
   } catch (e) {
     if (e instanceof TypeError && e.message.indexOf("open") > -1) {
       exceptionRaised = true;
diff --git a/toolkit/components/osfile/tests/mochi/worker_test_osfile_win.js b/toolkit/components/osfile/tests/mochi/worker_test_osfile_win.js
index 7aa1e304fa3b..2a8d829f228b 100644
--- a/toolkit/components/osfile/tests/mochi/worker_test_osfile_win.js
+++ b/toolkit/components/osfile/tests/mochi/worker_test_osfile_win.js
@@ -122,7 +122,7 @@ function test_ReadWrite() {
     }
     while (bytes_left.value > 0) {
       log("test_ReadWrite: writing " + bytes_left.value);
-      array.addressOfElement(write_from);
+      let ptr = array.addressOfElement(write_from);
       // Note: |WriteFile| launches an exception in case of error
       result = OS.Win.File.WriteFile(output, array, bytes_left, bytes_written_ptr, null);
       isnot(result, 0, "test_ReadWrite: write success");
@@ -193,7 +193,7 @@ function test_passing_undefined() {
   let exceptionRaised = false;
 
   try {
-    OS.Win.File.CreateFile(
+    let file = OS.Win.File.CreateFile(
       undefined,
       OS.Constants.Win.GENERIC_READ,
       0,
diff --git a/toolkit/components/osfile/tests/xpcshell/test_compression.js b/toolkit/components/osfile/tests/xpcshell/test_compression.js
index c0f479965257..f7a0f1c40e43 100644
--- a/toolkit/components/osfile/tests/xpcshell/test_compression.js
+++ b/toolkit/components/osfile/tests/xpcshell/test_compression.js
@@ -36,7 +36,7 @@ add_task(async function test_uncompressed() {
   for (let i = 0; i < array.byteLength; ++i) {
     array[i] = i;
   }
-  await OS.File.writeAtomic(path, array); // No compression
+  let bytes = await OS.File.writeAtomic(path, array); // No compression
 
   let exn;
   // Force decompression, reading should fail
@@ -56,7 +56,7 @@ add_task(async function test_no_header() {
 
   do_print("Writing data with no header");
 
-  await OS.File.writeAtomic(path, array); // No compression
+  let bytes = await OS.File.writeAtomic(path, array); // No compression
   let exn;
   // Force decompression, reading should fail
   try {
@@ -80,7 +80,7 @@ add_task(async function test_invalid_content() {
 
   do_print("Writing invalid data (with a valid header and only ones after that)");
 
-  await OS.File.writeAtomic(path, array); // No compression
+  let bytes = await OS.File.writeAtomic(path, array); // No compression
   let exn;
   // Force decompression, reading should fail
   try {
diff --git a/toolkit/components/osfile/tests/xpcshell/test_creationDate.js b/toolkit/components/osfile/tests/xpcshell/test_creationDate.js
index aae234b3ebb8..216827d4200e 100644
--- a/toolkit/components/osfile/tests/xpcshell/test_creationDate.js
+++ b/toolkit/components/osfile/tests/xpcshell/test_creationDate.js
@@ -15,7 +15,7 @@ add_task(async function test_deprecatedCreationDate() {
 
   let consoleMessagePromise = new Promise(resolve => {
     let consoleListener = {
-      observe(aMessage) {
+      observe: function(aMessage) {
         if (aMessage.message.indexOf("Field 'creationDate' is deprecated.") > -1) {
         do_print("Deprecation message printed");
           do_check_true(true);
diff --git a/toolkit/components/osfile/tests/xpcshell/test_duration.js b/toolkit/components/osfile/tests/xpcshell/test_duration.js
index d2026855a4a6..99fe93b6e4a8 100644
--- a/toolkit/components/osfile/tests/xpcshell/test_duration.js
+++ b/toolkit/components/osfile/tests/xpcshell/test_duration.js
@@ -59,7 +59,7 @@ add_task(async function duration() {
     // duration measurement then progressively incremented.
     outSerializationDuration: null,
     outExecutionDuration: null,
-    tmpPath
+    tmpPath: tmpPath
   };
   await OS.File.writeAtomic(pathDest, contents, writeAtomicOptions);
   testOptions(writeAtomicOptions, "OS.File.writeAtomic");
diff --git a/toolkit/components/osfile/tests/xpcshell/test_logging.js b/toolkit/components/osfile/tests/xpcshell/test_logging.js
index 70adb59404d6..9dd2282231bc 100644
--- a/toolkit/components/osfile/tests/xpcshell/test_logging.js
+++ b/toolkit/components/osfile/tests/xpcshell/test_logging.js
@@ -15,7 +15,7 @@ function run_test() {
 
   // Create a console listener.
   let consoleListener = {
-    observe(aMessage) {
+    observe: function(aMessage) {
       // Ignore unexpected messages.
       if (!(aMessage instanceof Components.interfaces.nsIConsoleMessage)) {
         return;
@@ -71,3 +71,4 @@ function run_test() {
   // Once both messages are observed OS.Shared.DEBUG, and OS.Shared.TEST
   // are reset to false.
 }
+
diff --git a/toolkit/components/osfile/tests/xpcshell/test_osfile_async_largefiles.js b/toolkit/components/osfile/tests/xpcshell/test_osfile_async_largefiles.js
index 16086f4c5fde..b1a9bfce14c1 100644
--- a/toolkit/components/osfile/tests/xpcshell/test_osfile_async_largefiles.js
+++ b/toolkit/components/osfile/tests/xpcshell/test_osfile_async_largefiles.js
@@ -62,6 +62,8 @@ async function test_setPosition_failures() {
   try {
     let file = await OS.File.open(path, {write: true, append: false});
     try {
+      let pos = 0;
+
       // 1. Use an invalid position value
       try {
         await file.setPosition(0.5, OS.File.POS_START);
@@ -112,7 +114,7 @@ function run_test() {
   add_task(test_setPosition.bind(null, 1000, 100, 50));
   add_task(test_setPosition.bind(null, 1000, -100, -50));
 
-  if (OS.Constants.Win || ctypes.off_t.size >= 8) {
+  if (OS.Constants.Win || ctypes.off_t.size >= 8) {
     // Now verify stuff still works for large values.
     // 1. Multiple small seeks, which add up to > MAXINT32
     add_task(test_setPosition.bind(null, 0x7fffffff, 0x7fffffff, 0));
diff --git a/toolkit/components/osfile/tests/xpcshell/test_osfile_async_setDates.js b/toolkit/components/osfile/tests/xpcshell/test_osfile_async_setDates.js
index bb72d5daded3..7a072d1c6c60 100644
--- a/toolkit/components/osfile/tests/xpcshell/test_osfile_async_setDates.js
+++ b/toolkit/components/osfile/tests/xpcshell/test_osfile_async_setDates.js
@@ -1,7 +1,5 @@
 "use strict";
 
-/* eslint-disable no-lone-blocks */
-
 Components.utils.import("resource://gre/modules/osfile.jsm");
 
 /**
@@ -108,7 +106,7 @@ add_task(async function test_proto() {
                               "test_osfile_async_setDates_proto.tmp");
   await OS.File.writeAtomic(path, new Uint8Array(1));
 
-  try {
+  try {
     let fd = await OS.File.open(path, {write: true});
 
     try {
diff --git a/toolkit/components/osfile/tests/xpcshell/test_osfile_kill.js b/toolkit/components/osfile/tests/xpcshell/test_osfile_kill.js
index ddf04b44b90a..8e94e5ace794 100644
--- a/toolkit/components/osfile/tests/xpcshell/test_osfile_kill.js
+++ b/toolkit/components/osfile/tests/xpcshell/test_osfile_kill.js
@@ -62,7 +62,7 @@ add_task(async function test_kill_race() {
   await OS.File.exists("foo.foo");
 
   do_print("issuing first request");
-  let firstRequest = OS.File.exists("foo.bar"); // eslint-disable-line no-unused-vars
+  let firstRequest = OS.File.exists("foo.bar");
   let secondRequest;
   let secondResolved = false;
 
diff --git a/toolkit/components/osfile/tests/xpcshell/test_osfile_writeAtomic_backupTo_option.js b/toolkit/components/osfile/tests/xpcshell/test_osfile_writeAtomic_backupTo_option.js
index 5f78955be398..02133f069e4c 100644
--- a/toolkit/components/osfile/tests/xpcshell/test_osfile_writeAtomic_backupTo_option.js
+++ b/toolkit/components/osfile/tests/xpcshell/test_osfile_writeAtomic_backupTo_option.js
@@ -103,6 +103,7 @@ add_task(async function test_non_backupTo_option() {
  * @result no back up file exists
  */
 add_task(async function test_backupTo_option_without_destination_file() {
+  let DEFAULT_CONTENTS = "default contents" + Math.random();
   let WRITE_CONTENTS = "abc" + Math.random();
   let path = Path.join(Constants.Path.tmpDir,
                        "test_backupTo_option_without_destination_file.tmp");
diff --git a/toolkit/components/osfile/tests/xpcshell/test_queue.js b/toolkit/components/osfile/tests/xpcshell/test_queue.js
index 5ad16f1205ff..bdd2bc71dea9 100644
--- a/toolkit/components/osfile/tests/xpcshell/test_queue.js
+++ b/toolkit/components/osfile/tests/xpcshell/test_queue.js
@@ -16,7 +16,7 @@ add_task(async function check_init() {
 // after an operation is successful.
 add_task(async function check_success() {
   do_print("Attempting to open a file correctly");
-  await OS.File.open(OS.Path.join(do_get_cwd().path, "test_queue.js"));
+  let openedFile = await OS.File.open(OS.Path.join(do_get_cwd().path, "test_queue.js"));
   do_print("File opened correctly");
   await OS.File.queue;
   do_print("Function resolved");
diff --git a/toolkit/components/osfile/tests/xpcshell/test_read_write.js b/toolkit/components/osfile/tests/xpcshell/test_read_write.js
index da33fe5dfe75..227010329fff 100644
--- a/toolkit/components/osfile/tests/xpcshell/test_read_write.js
+++ b/toolkit/components/osfile/tests/xpcshell/test_read_write.js
@@ -86,7 +86,7 @@ add_test_pair(async function read_write_all() {
 
       let array2 = await OS.File.read(DEST_PATH);
       do_check_eq(LENGTH, array2.length);
-      for (var j = 0; j < LENGTH; j++)
+      for (var i = 0; i < LENGTH; i++)
         do_check_eq(array2[i], (i + START) % 256);
 
       // Cleanup.