diff --git a/extensions/firefox/content/PdfJs.jsm b/extensions/firefox/content/PdfJs.jsm
index 50ee8adcc..263186b54 100644
--- a/extensions/firefox/content/PdfJs.jsm
+++ b/extensions/firefox/content/PdfJs.jsm
@@ -67,10 +67,11 @@ function getIntPref(aPref, aDefaultValue) {
 }
 
 function isDefaultHandler() {
- if (Services.appinfo.processType === Services.appinfo.PROCESS_TYPE_CONTENT) {
-   return PdfjsContentUtils.isDefaultHandlerApp();
- }
- return PdfjsChromeUtils.isDefaultHandlerApp();
+  if (Services.appinfo.processType !== Services.appinfo.PROCESS_TYPE_DEFAULT) {
+    throw new Error("isDefaultHandler should only get called in the parent " +
+                    "process.");
+  }
+  return PdfjsChromeUtils.isDefaultHandlerApp();
 }
 
 function initializeDefaultPreferences() {
@@ -251,13 +252,16 @@ var PdfJs = {
 
   // nsIObserver
   observe: function observe(aSubject, aTopic, aData) {
-    this.updateRegistration();
-    if (Services.appinfo.processType ===
+    if (Services.appinfo.processType !==
         Services.appinfo.PROCESS_TYPE_DEFAULT) {
-      let jsm = "resource://pdf.js/PdfjsChromeUtils.jsm";
-      let PdfjsChromeUtils = Components.utils.import(jsm, {}).PdfjsChromeUtils;
-      PdfjsChromeUtils.notifyChildOfSettingsChange();
+      throw new Error("Only the parent process should be observing PDF " +
+                      "handler changes.");
     }
+
+    this.updateRegistration();
+    let jsm = "resource://pdf.js/PdfjsChromeUtils.jsm";
+    let PdfjsChromeUtils = Components.utils.import(jsm, {}).PdfjsChromeUtils;
+    PdfjsChromeUtils.notifyChildOfSettingsChange(this.enabled);
   },
 
   /**
diff --git a/extensions/firefox/content/PdfjsChromeUtils.jsm b/extensions/firefox/content/PdfjsChromeUtils.jsm
index b22ade9d9..af7b4c2fb 100644
--- a/extensions/firefox/content/PdfjsChromeUtils.jsm
+++ b/extensions/firefox/content/PdfjsChromeUtils.jsm
@@ -117,7 +117,7 @@ var PdfjsChromeUtils = {
    * instruct the child to refresh its configuration and (possibly)
    * the module's registration.
    */
-  notifyChildOfSettingsChange() {
+  notifyChildOfSettingsChange(enabled) {
     if (Services.appinfo.processType ===
         Services.appinfo.PROCESS_TYPE_DEFAULT && this._ppmm) {
       // XXX kinda bad, we want to get the parent process mm associated
@@ -125,7 +125,8 @@ var PdfjsChromeUtils = {
       // manager, which means this is going to fire to every child process
       // we have open. Unfortunately I can't find a way to get at that
       // process specific mm from js.
-      this._ppmm.broadcastAsyncMessage("PDFJS:Child:refreshSettings", {});
+      this._ppmm.broadcastAsyncMessage("PDFJS:Child:updateSettings",
+                                       { enabled, });
     }
   },
 
diff --git a/extensions/firefox/content/PdfjsContentUtils.jsm b/extensions/firefox/content/PdfjsContentUtils.jsm
index e625c4528..1d55ffc16 100644
--- a/extensions/firefox/content/PdfjsContentUtils.jsm
+++ b/extensions/firefox/content/PdfjsContentUtils.jsm
@@ -43,7 +43,7 @@ var PdfjsContentUtils = {
     if (!this._mm) {
       this._mm = Cc["@mozilla.org/childprocessmessagemanager;1"].
         getService(Ci.nsISyncMessageSender);
-      this._mm.addMessageListener("PDFJS:Child:refreshSettings", this);
+      this._mm.addMessageListener("PDFJS:Child:updateSettings", this);
 
 //#if !MOZCENTRAL
       // The signature of `Services.obs.addObserver` changed in Firefox 55,
@@ -62,7 +62,7 @@ var PdfjsContentUtils = {
 
   uninit() {
     if (this._mm) {
-      this._mm.removeMessageListener("PDFJS:Child:refreshSettings", this);
+      this._mm.removeMessageListener("PDFJS:Child:updateSettings", this);
       Services.obs.removeObserver(this, "quit-application");
     }
     this._mm = null;
@@ -108,14 +108,6 @@ var PdfjsContentUtils = {
     });
   },
 
-  /*
-   * Forwards default app query to the parent where we check various
-   * handler app settings only available in the parent process.
-   */
-  isDefaultHandlerApp() {
-    return this._mm.sendSyncMessage("PDFJS:Parent:isDefaultHandlerApp")[0];
-  },
-
   /*
    * Request the display of a notification warning in the associated window
    * when the renderer isn't sure a pdf displayed correctly.
@@ -145,13 +137,17 @@ var PdfjsContentUtils = {
 
   receiveMessage(aMsg) {
     switch (aMsg.name) {
-      case "PDFJS:Child:refreshSettings":
+      case "PDFJS:Child:updateSettings":
         // Only react to this if we are remote.
         if (Services.appinfo.processType ===
             Services.appinfo.PROCESS_TYPE_CONTENT) {
           let jsm = "resource://pdf.js/PdfJs.jsm";
           let pdfjs = Components.utils.import(jsm, {}).PdfJs;
-          pdfjs.updateRegistration();
+          if (aMsg.data.enabled) {
+            pdfjs.ensureRegistered();
+          } else {
+            pdfjs.ensureUnregistered();
+          }
         }
         break;
     }