diff --git a/toolkit/components/osfile/modules/ospath_unix.jsm b/toolkit/components/osfile/modules/ospath_unix.jsm
index 35d19b103611..a040459549ed 100644
--- a/toolkit/components/osfile/modules/ospath_unix.jsm
+++ b/toolkit/components/osfile/modules/ospath_unix.jsm
@@ -20,6 +20,7 @@
 // Boilerplate used to be able to import this module both from the main
 // thread and from worker threads.
 if (typeof Components != "undefined") {
+  Components.utils.importGlobalProperties(["URL"]);
   // Global definition of |exports|, to keep everybody happy.
   // In non-main thread, |exports| is provided by the module
   // loader.
@@ -33,7 +34,9 @@ let EXPORTED_SYMBOLS = [
   "dirname",
   "join",
   "normalize",
-  "split"
+  "split",
+  "toFileURI",
+  "fromFileURI",
 ];
 
 /**
@@ -148,6 +151,37 @@ let split = function(path) {
 };
 exports.split = split;
 
+/**
+ * Returns the file:// URI file path of the given local file path.
+ */
+// The case of %3b is designed to match Services.io, but fundamentally doesn't matter.
+let toFileURIExtraEncodings = {';': '%3b', '?': '%3F', "'": '%27', '#': '%23'};
+let toFileURI = function toFileURI(path) {
+  let uri = encodeURI(this.normalize(path));
+
+  // add a prefix, and encodeURI doesn't escape a few characters that we do
+  // want to escape, so fix that up
+  let prefix = "file://";
+  uri = prefix + uri.replace(/[;?'#]/g, match => toFileURIExtraEncodings[match]);
+
+  return uri;
+};
+exports.toFileURI = toFileURI;
+
+/**
+ * Returns the local file path from a given file URI.
+ */
+let fromFileURI = function fromFileURI(uri) {
+  let url = new URL(uri);
+  if (url.protocol != 'file:') {
+    throw new Error("fromFileURI expects a file URI");
+  }
+  let path = this.normalize(decodeURIComponent(url.pathname));
+  return path;
+};
+exports.fromFileURI = fromFileURI;
+
+
 //////////// Boilerplate
 if (typeof Components != "undefined") {
   this.EXPORTED_SYMBOLS = EXPORTED_SYMBOLS;
diff --git a/toolkit/components/osfile/modules/ospath_win.jsm b/toolkit/components/osfile/modules/ospath_win.jsm
index 45cc7bd507a2..bf3079638442 100644
--- a/toolkit/components/osfile/modules/ospath_win.jsm
+++ b/toolkit/components/osfile/modules/ospath_win.jsm
@@ -29,6 +29,7 @@
 // Boilerplate used to be able to import this module both from the main
 // thread and from worker threads.
 if (typeof Components != "undefined") {
+  Components.utils.importGlobalProperties(["URL"]);
   // Global definition of |exports|, to keep everybody happy.
   // In non-main thread, |exports| is provided by the module
   // loader.
@@ -44,7 +45,9 @@ let EXPORTED_SYMBOLS = [
   "normalize",
   "split",
   "winGetDrive",
-  "winIsAbsolute"
+  "winIsAbsolute",
+  "toFileURI",
+  "fromFileURI",
 ];
 
 /**
@@ -287,6 +290,57 @@ let split = function(path) {
 };
 exports.split = split;
 
+/**
+ * Return the file:// URI file path of the given local file path.
+ */
+// The case of %3b is designed to match Services.io, but fundamentally doesn't matter.
+let toFileURIExtraEncodings = {';': '%3b', '?': '%3F', "'": '%27', '#': '%23'};
+let toFileURI = function toFileURI(path) {
+  // URI-escape forward slashes and convert backward slashes to forward
+  path = this.normalize(path).replace(/[\\\/]/g, m => (m=='\\')? '/' : '%2F');
+  let uri = encodeURI(path);
+
+  // add a prefix, and encodeURI doesn't escape a few characters that we do
+  // want to escape, so fix that up
+  let prefix = "file:///";
+  uri = prefix + uri.replace(/[;?'#]/g, match => toFileURIExtraEncodings[match]);
+
+  // turn e.g., file:///C: into file:///C:/
+  if (uri.charAt(uri.length - 1) === ':') {
+    uri += "/"
+  }
+
+  return uri;
+};
+exports.toFileURI = toFileURI;
+
+/**
+ * Returns the local file path from a given file URI.
+ */
+let fromFileURI = function fromFileURI(uri) {
+  let url = new URL(uri);
+  if (url.protocol != 'file:') {
+    throw new Error("fromFileURI expects a file URI");
+  }
+
+  // strip leading slash, since Windows paths don't start with one
+  uri = url.pathname.substr(1);
+
+  let path = decodeURI(uri);
+  // decode a few characters where URL's parsing is overzealous
+  path = path.replace(/%(3b|3f|23)/gi,
+        match => decodeURIComponent(match));
+  path = this.normalize(path);
+
+  // this.normalize() does not remove the trailing slash if the path
+  // component is a drive letter. eg. 'C:\'' will not get normalized.
+  if (path.endsWith(":\\")) {
+    path = path.substr(0, path.length - 1);
+  }
+  return this.normalize(path);
+};
+exports.fromFileURI = fromFileURI;
+
 /**
 * Utility function: Remove any leading/trailing backslashes
 * from a string.
diff --git a/toolkit/components/osfile/tests/xpcshell/test_file_URL_conversion.js b/toolkit/components/osfile/tests/xpcshell/test_file_URL_conversion.js
new file mode 100644
index 000000000000..9802a2a9d9ea
--- /dev/null
+++ b/toolkit/components/osfile/tests/xpcshell/test_file_URL_conversion.js
@@ -0,0 +1,111 @@
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this file,
+ * You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+function run_test() {
+  Components.utils.import("resource://gre/modules/Services.jsm");
+  Components.utils.import("resource://gre/modules/osfile.jsm");
+  Components.utils.import("resource://gre/modules/FileUtils.jsm");
+
+  let isWindows = ("@mozilla.org/windows-registry-key;1" in Components.classes);
+
+  // Test cases for filePathToURI
+  let paths = isWindows ? [
+    'C:\\',
+    'C:\\test',
+    'C:\\test\\',
+    'C:\\test%2f',
+    'C:\\test\\test\\test',
+    'C:\\test;+%',
+    'C:\\test?action=index\\',
+    'C:\\test\ test',
+    '\\\\C:\\a\\b\\c',
+    '\\\\Server\\a\\b\\c',
+
+    // note that per http://support.microsoft.com/kb/177506 (under more info),
+    // the following characters are allowed on Windows:
+    'C:\\char^',
+    'C:\\char&',
+    'C:\\char\'',
+    'C:\\char@',
+    'C:\\char{',
+    'C:\\char}',
+    'C:\\char[',
+    'C:\\char]',
+    'C:\\char,',
+    'C:\\char$',
+    'C:\\char=',
+    'C:\\char!',
+    'C:\\char-',
+    'C:\\char#',
+    'C:\\char(',
+    'C:\\char)',
+    'C:\\char%',
+    'C:\\char.',
+    'C:\\char+',
+    'C:\\char~',
+    'C:\\char_'
+  ] : [
+    '/',
+    '/test',
+    '/test/',
+    '/test%2f',
+    '/test/test/test',
+    '/test;+%',
+    '/test?action=index/',
+    '/test\ test',
+    '/punctuation/;,/?:@&=+$-_.!~*\'()"#',
+    '/CasePreserving'
+  ];
+
+  // some additional URIs to test, beyond those generated from paths
+  let uris = isWindows ? [
+    'file:///C:/test/',
+    'file://localhost/C:/test',
+    'file:///c:/test/test.txt',
+    //'file:///C:/foo%2f', // trailing, encoded slash
+    'file:///C:/%3f%3F',
+    'file:///C:/%3b%3B',
+    'file:///C:/%3c%3C', // not one of the special-cased ? or ;
+    'file:///C:/%78', // 'x', not usually uri encoded
+    'file:///C:/test#frag', // a fragment identifier
+    'file:///C:/test?action=index' // an actual query component
+  ] : [
+    'file:///test/',
+    'file://localhost/test',
+    'file:///test/test.txt',
+    'file:///foo%2f', // trailing, encoded slash
+    'file:///%3f%3F',
+    'file:///%3b%3B',
+    'file:///%3c%3C', // not one of the special-cased ? or ;
+    'file:///%78', // 'x', not usually uri encoded
+    'file:///test#frag', // a fragment identifier
+    'file:///test?action=index' // an actual query component
+  ];
+
+  for (let path of paths) {
+    // convert that to a uri using FileUtils and Services, which toFileURI is trying to model
+    let file = FileUtils.File(path);
+    let uri = Services.io.newFileURI(file).spec;
+    do_check_eq(uri, OS.Path.toFileURI(path));
+
+    // keep the resulting URI to try the reverse
+    uris.push(uri)
+  }
+
+  for (let uri of uris) {
+    // convert URIs to paths with nsIFileURI, which fromFileURI is trying to model
+    let path = Services.io.newURI(uri, null, null).QueryInterface(Components.interfaces.nsIFileURL).file.path;
+    do_check_eq(path, OS.Path.fromFileURI(uri));
+  }
+
+  // check that non-file URLs aren't allowed
+  let thrown = false;
+  try {
+    OS.Path.fromFileURI('http://test.com')
+  } catch (e) {
+    do_check_eq(e.message, "fromFileURI expects a file URI");
+    thrown = true;
+  }
+  do_check_true(thrown);
+}
diff --git a/toolkit/components/osfile/tests/xpcshell/xpcshell.ini b/toolkit/components/osfile/tests/xpcshell/xpcshell.ini
index 19646c017c28..29bc2b02e049 100644
--- a/toolkit/components/osfile/tests/xpcshell/xpcshell.ini
+++ b/toolkit/components/osfile/tests/xpcshell/xpcshell.ini
@@ -13,6 +13,7 @@ tail =
 [test_removeEmptyDir.js]
 [test_makeDir.js]
 [test_profiledir.js]
+[test_file_URL_conversion.js]
 [test_logging.js]
 [test_creationDate.js]
 [test_exception.js]