diff --git a/browser/components/BrowserGlue.jsm b/browser/components/BrowserGlue.jsm
index adb3337de40a..9bd25e88c900 100644
--- a/browser/components/BrowserGlue.jsm
+++ b/browser/components/BrowserGlue.jsm
@@ -62,7 +62,7 @@ XPCOMUtils.defineLazyServiceGetter(
   "nsIPushService"
 );
 
-const PREF_PDFJS_ENABLED_CACHE_STATE = "pdfjs.enabledCache.state";
+const PREF_PDFJS_ISDEFAULT_CACHE_STATE = "pdfjs.enabledCache.state";
 
 /**
  * Fission-compatible JSWindowActor implementations.
@@ -1282,13 +1282,8 @@ BrowserGlue.prototype = {
     // handle any UI migration
     this._migrateUI();
 
-    if (Services.prefs.prefHasUserValue(PREF_PDFJS_ENABLED_CACHE_STATE)) {
-      Services.ppmm.sharedData.set(
-        "pdfjs.enabled",
-        Services.prefs.getBoolPref(PREF_PDFJS_ENABLED_CACHE_STATE)
-      );
-    } else {
-      PdfJs.earlyInit(this._isNewProfile);
+    if (!Services.prefs.prefHasUserValue(PREF_PDFJS_ISDEFAULT_CACHE_STATE)) {
+      PdfJs.checkIsDefault(this._isNewProfile);
     }
 
     listeners.init();
diff --git a/browser/components/preferences/main.js b/browser/components/preferences/main.js
index 2e513d2e8fe8..9f36871d6303 100644
--- a/browser/components/preferences/main.js
+++ b/browser/components/preferences/main.js
@@ -49,7 +49,6 @@ XPCOMUtils.defineLazyServiceGetters(this, {
 const TYPE_PDF = "application/pdf";
 
 const PREF_PDFJS_DISABLED = "pdfjs.disabled";
-const TOPIC_PDFJS_HANDLER_CHANGED = "pdfjs:handlerChanged";
 
 const PREF_DISABLED_PLUGIN_TYPES = "plugin.disable_full_page_plugin_for_types";
 
@@ -3648,7 +3647,6 @@ class InternalHandlerInfoWrapper extends HandlerInfoWrapper {
   // or unregistration of this handler.
   store() {
     super.store();
-    Services.obs.notifyObservers(null, this._handlerChanged);
   }
 
   get enabled() {
@@ -3665,10 +3663,6 @@ class PDFHandlerInfoWrapper extends InternalHandlerInfoWrapper {
     super(TYPE_PDF);
   }
 
-  get _handlerChanged() {
-    return TOPIC_PDFJS_HANDLER_CHANGED;
-  }
-
   get _appPrefLabel() {
     return "applications-type-pdf";
   }
diff --git a/browser/extensions/pdfjs/content/PdfJs.jsm b/browser/extensions/pdfjs/content/PdfJs.jsm
index 21b07abd8f45..cc87df1ffec3 100644
--- a/browser/extensions/pdfjs/content/PdfJs.jsm
+++ b/browser/extensions/pdfjs/content/PdfJs.jsm
@@ -23,7 +23,7 @@ const PREF_MIGRATION_VERSION = PREF_PREFIX + ".migrationVersion";
 const PREF_PREVIOUS_ACTION = PREF_PREFIX + ".previousHandler.preferredAction";
 const PREF_PREVIOUS_ASK =
   PREF_PREFIX + ".previousHandler.alwaysAskBeforeHandling";
-const PREF_ENABLED_CACHE_STATE = PREF_PREFIX + ".enabledCache.state";
+const PREF_ISDEFAULT_CACHE_STATE = PREF_PREFIX + ".enabledCache.state";
 const TOPIC_PDFJS_HANDLER_CHANGED = "pdfjs:handlerChanged";
 const PDF_CONTENT_TYPE = "application/pdf";
 
@@ -98,6 +98,7 @@ const gPdfFakeHandlerInfo = {
 var PdfJs = {
   QueryInterface: ChromeUtils.generateQI([Ci.nsIObserver]),
   _initialized: false,
+  _cachedIsDefault: true,
 
   init: function init(isNewProfile) {
     if (
@@ -109,20 +110,7 @@ var PdfJs = {
     }
     PdfjsChromeUtils.init();
     this.initPrefs();
-
-    Services.ppmm.sharedData.set(
-      "pdfjs.enabled",
-      this.checkEnabled(isNewProfile)
-    );
-  },
-
-  earlyInit(isNewProfile) {
-    // Note: Please keep this in sync with the duplicated logic in
-    // BrowserGlue.jsm.
-    Services.ppmm.sharedData.set(
-      "pdfjs.enabled",
-      this.checkEnabled(isNewProfile)
-    );
+    this.checkIsDefault(isNewProfile);
   },
 
   initPrefs: function initPrefs() {
@@ -131,15 +119,13 @@ var PdfJs = {
     }
     this._initialized = true;
 
-    if (Services.prefs.getBoolPref(PREF_DISABLED, false)) {
+    if (this._prefDisabled) {
       this._unbecomeHandler();
     } else {
       this._migrate();
     }
 
-    // Listen for when pdf.js is completely disabled or a different pdf handler
-    // is chosen.
-    Services.prefs.addObserver(PREF_DISABLED, this);
+    // Listen for when a different pdf handler is chosen.
     Services.obs.addObserver(this, TOPIC_PDFJS_HANDLER_CHANGED);
 
     initializeDefaultPreferences();
@@ -147,7 +133,6 @@ var PdfJs = {
 
   uninit: function uninit() {
     if (this._initialized) {
-      Services.prefs.removeObserver(PREF_DISABLED, this);
       Services.obs.removeObserver(this, TOPIC_PDFJS_HANDLER_CHANGED);
       this._initialized = false;
     }
@@ -232,15 +217,15 @@ var PdfJs = {
    *                     pdfs differently. If we're on a new profile,
    *                     there's no need to check.
    */
-  _isEnabled(isNewProfile) {
+  _isDefault(isNewProfile) {
     let { processType, PROCESS_TYPE_DEFAULT } = Services.appinfo;
     if (processType !== PROCESS_TYPE_DEFAULT) {
       throw new Error(
-        "isEnabled should only get called in the parent process."
+        "isDefault should only get called in the parent process."
       );
     }
 
-    if (Services.prefs.getBoolPref(PREF_DISABLED, true)) {
+    if (this._prefDisabled) {
       return false;
     }
 
@@ -252,28 +237,32 @@ var PdfJs = {
     let handlerInfo = Svc.mime.getFromTypeAndExtension(PDF_CONTENT_TYPE, "pdf");
     return (
       !handlerInfo.alwaysAskBeforeHandling &&
-      handlerInfo.preferredAction === Ci.nsIHandlerInfo.handleInternally
+      handlerInfo.preferredAction == Ci.nsIHandlerInfo.handleInternally
     );
   },
 
-  checkEnabled(isNewProfile) {
-    let isEnabled = this._isEnabled(isNewProfile);
-    // This will be updated any time we observe a dependency changing, since
-    // updateRegistration internally calls enabled.
-    Services.prefs.setBoolPref(PREF_ENABLED_CACHE_STATE, isEnabled);
-    return isEnabled;
+  checkIsDefault(isNewProfile) {
+    this._cachedIsDefault = this._isDefault(isNewProfile);
+    Services.prefs.setBoolPref(
+      PREF_ISDEFAULT_CACHE_STATE,
+      this._cachedIsDefault
+    );
+  },
+
+  cachedIsDefault() {
+    return this._cachedIsDefault;
   },
 
   // nsIObserver
-  observe: function observe(aSubject, aTopic, aData) {
-    if (
-      Services.appinfo.processType !== Services.appinfo.PROCESS_TYPE_DEFAULT
-    ) {
-      throw new Error(
-        "Only the parent process should be observing PDF handler changes."
-      );
-    }
-
-    Services.ppmm.sharedData.set("pdfjs.enabled", this.checkEnabled());
+  observe(aSubject, aTopic, aData) {
+    this.checkIsDefault();
   },
 };
+
+XPCOMUtils.defineLazyPreferenceGetter(
+  PdfJs,
+  "_prefDisabled",
+  PREF_DISABLED,
+  false,
+  () => PdfJs.checkIsDefault()
+);
diff --git a/browser/extensions/pdfjs/content/PdfStreamConverter.jsm b/browser/extensions/pdfjs/content/PdfStreamConverter.jsm
index 69a53f5b7afe..7e1b617a1210 100644
--- a/browser/extensions/pdfjs/content/PdfStreamConverter.jsm
+++ b/browser/extensions/pdfjs/content/PdfStreamConverter.jsm
@@ -23,11 +23,15 @@ const PDF_VIEWER_ORIGIN = "resource://pdf.js";
 const PDF_VIEWER_WEB_PAGE = "resource://pdf.js/web/viewer.html";
 const MAX_NUMBER_OF_PREFS = 50;
 const MAX_STRING_PREF_LENGTH = 128;
+const PDF_CONTENT_TYPE = "application/pdf";
 
 const { XPCOMUtils } = ChromeUtils.import(
   "resource://gre/modules/XPCOMUtils.jsm"
 );
 const { Services } = ChromeUtils.import("resource://gre/modules/Services.jsm");
+const { AppConstants } = ChromeUtils.import(
+  "resource://gre/modules/AppConstants.jsm"
+);
 
 ChromeUtils.defineModuleGetter(
   this,
@@ -58,6 +62,7 @@ ChromeUtils.defineModuleGetter(
   "PdfjsContentUtils",
   "resource://pdf.js/PdfjsContentUtils.jsm"
 );
+ChromeUtils.defineModuleGetter(this, "PdfJs", "resource://pdf.js/PdfJs.jsm");
 
 XPCOMUtils.defineLazyGlobalGetters(this, ["XMLHttpRequest"]);
 
@@ -68,6 +73,26 @@ XPCOMUtils.defineLazyServiceGetter(
   "@mozilla.org/mime;1",
   "nsIMIMEService"
 );
+XPCOMUtils.defineLazyServiceGetter(
+  Svc,
+  "handlers",
+  "@mozilla.org/uriloader/handler-service;1",
+  "nsIHandlerService"
+);
+
+XPCOMUtils.defineLazyGetter(this, "gOurBinary", () => {
+  let file = Services.dirsvc.get("XREExeF", Ci.nsIFile);
+  // Make sure to get the .app on macOS
+  if (AppConstants.platform == "macosx") {
+    while (file) {
+      if (/\.app\/?$/i.test(file.leafName)) {
+        break;
+      }
+      file = file.parent;
+    }
+  }
+  return file;
+});
 
 function getBoolPref(pref, def) {
   try {
@@ -326,7 +351,7 @@ class ChromeActions {
             Ci.nsILoadContext
           );
           this.extListener = extHelperAppSvc.doContent(
-            data.isAttachment ? "application/octet-stream" : "application/pdf",
+            data.isAttachment ? "application/octet-stream" : PDF_CONTENT_TYPE,
             aRequest,
             loadContext,
             false
@@ -1010,7 +1035,86 @@ PdfStreamConverter.prototype = {
     this.listener = aListener;
   },
 
+  _usableHandler(handlerInfo) {
+    let { preferredApplicationHandler } = handlerInfo;
+    if (
+      !preferredApplicationHandler ||
+      !(preferredApplicationHandler instanceof Ci.nsILocalHandlerApp)
+    ) {
+      return false;
+    }
+    preferredApplicationHandler.QueryInterface(Ci.nsILocalHandlerApp);
+    // We have an app, grab the executable
+    let { executable } = preferredApplicationHandler;
+    if (!executable) {
+      return false;
+    }
+    return !executable.equals(gOurBinary);
+  },
+
+  /*
+   * Check if the user wants to use PDF.js. Returns true if PDF.js should
+   * handle PDFs, and false if not. Will always return true on non-parent
+   * processes.
+   *
+   * If the user has selected to open PDFs with a helper app, and we are that
+   * helper app, or if the user has selected the OS default, and we are that
+   * OS default, reset the preference back to pdf.js .
+   *
+   */
+  _validateAndMaybeUpdatePDFPrefs() {
+    let { processType, PROCESS_TYPE_DEFAULT } = Services.appinfo;
+    // If we're not in the parent, or are the default, then just say yes.
+    if (processType != PROCESS_TYPE_DEFAULT || PdfJs.cachedIsDefault()) {
+      return true;
+    }
+
+    // OK, PDF.js might not be the default. Find out if we've misled the user
+    // into making Firefox an external handler or if we're the OS default and
+    // Firefox is set to use the OS default:
+    let mime = Svc.mime.getFromTypeAndExtension(PDF_CONTENT_TYPE, "pdf");
+    // The above might throw errors. We're deliberately letting those bubble
+    // back up, where they'll tell the stream converter not to use us.
+
+    if (!mime) {
+      // This shouldn't happen, but we can't fix what isn't there. Assume
+      // we're OK to handle with PDF.js
+      return true;
+    }
+
+    const { saveToDisk, useHelperApp, useSystemDefault } = Ci.nsIHandlerInfo;
+    let { preferredAction, alwaysAskBeforeHandling } = mime;
+    // If the user has indicated they want to be asked or want to save to
+    // disk, we shouldn't render inline immediately:
+    if (alwaysAskBeforeHandling || preferredAction == saveToDisk) {
+      return false;
+    }
+    // If we have usable helper app info, don't use PDF.js
+    if (preferredAction == useHelperApp && this._usableHandler(mime)) {
+      return false;
+    }
+    // If we want the OS default and that's not Firefox, don't use PDF.js
+    if (preferredAction == useSystemDefault && !mime.isCurrentAppOSDefault()) {
+      return false;
+    }
+    // Log that we're doing this to help debug issues if people end up being
+    // surprised by this behaviour.
+    Cu.reportError("Found unusable PDF preferences. Fixing back to PDF.js");
+
+    mime.preferredAction = Ci.nsIHandlerInfo.handleInternally;
+    mime.alwaysAskBeforeHandling = false;
+    Svc.handlers.store(mime);
+    return true;
+  },
+
   getConvertedType(aFromType) {
+    if (!this._validateAndMaybeUpdatePDFPrefs()) {
+      throw new Components.Exception(
+        "Can't use PDF.js",
+        Cr.NS_ERROR_NOT_AVAILABLE
+      );
+    }
+
     return "text/html";
   },
 
diff --git a/browser/extensions/pdfjs/pdfjs.js b/browser/extensions/pdfjs/pdfjs.js
index 8114d7df1774..f19ad2227a81 100644
--- a/browser/extensions/pdfjs/pdfjs.js
+++ b/browser/extensions/pdfjs/pdfjs.js
@@ -25,7 +25,7 @@ ChromeUtils.defineModuleGetter(
 
 // Register/unregister a constructor as a factory.
 function StreamConverterFactory() {
-  if (Services.cpmm.sharedData.get("pdfjs.enabled")) {
+  if (!Services.prefs.getBoolPref("pdfjs.disabled", false)) {
     return new PdfStreamConverter();
   }
   throw Components.Exception("", Cr.NS_ERROR_FACTORY_NOT_REGISTERED);
diff --git a/browser/extensions/pdfjs/test/browser_pdfjs_savedialog.js b/browser/extensions/pdfjs/test/browser_pdfjs_savedialog.js
index 7095331638a7..48e0d64d7201 100644
--- a/browser/extensions/pdfjs/test/browser_pdfjs_savedialog.js
+++ b/browser/extensions/pdfjs/test/browser_pdfjs_savedialog.js
@@ -40,9 +40,6 @@ function changeMimeHandler(preferredAction, alwaysAskBeforeHandling) {
   handlerInfo.preferredAction = preferredAction;
   handlerService.store(handlerInfo);
 
-  Services.obs.notifyObservers(null, "pdfjs:handlerChanged");
-  Services.ppmm.sharedData.flush();
-
   // Refresh data
   handlerInfo = mimeService.getFromTypeAndExtension("application/pdf", "pdf");
 
diff --git a/uriloader/exthandler/HandlerService.js b/uriloader/exthandler/HandlerService.js
index 304c7ce71c37..62d866e7e09e 100644
--- a/uriloader/exthandler/HandlerService.js
+++ b/uriloader/exthandler/HandlerService.js
@@ -8,6 +8,8 @@ const { XPCOMUtils } = ChromeUtils.import(
 );
 const { Services } = ChromeUtils.import("resource://gre/modules/Services.jsm");
 
+const TOPIC_PDFJS_HANDLER_CHANGED = "pdfjs:handlerChanged";
+
 ChromeUtils.defineModuleGetter(
   this,
   "FileUtils",
@@ -443,6 +445,12 @@ HandlerService.prototype = {
     delete storedHandlerInfo.stubEntry;
 
     this._store.saveSoon();
+
+    // Now notify PDF.js. This is hacky, but a lot better than expecting all
+    // the consumers to do it...
+    if (handlerInfo.type == "application/pdf") {
+      Services.obs.notifyObservers(null, TOPIC_PDFJS_HANDLER_CHANGED);
+    }
   },
 
   // nsIHandlerService