--- a/browser/components/nsBrowserGlue.js
+++ b/browser/components/nsBrowserGlue.js
@@ -826,40 +826,16 @@ BrowserGlue.prototype = {
       },
     ];
 
     let nb = win.document.getElementById("high-priority-global-notificationbox");
     nb.appendNotification(message, "unsigned-addons-disabled", "",
                           nb.PRIORITY_WARNING_MEDIUM, buttons);
   },
 
-  _notifyDisabledNonMpc() {
-    let win = RecentWindow.getMostRecentBrowserWindow();
-    if (!win)
-      return;
-
-    // This is only going to be on Nightly and only for the 55 and 56
-    // cycles, and it points to a wiki page that is not localized, so
-    // no need to localize the message here...
-    let message = "Due to performance testing, we have disabled some of your add-ons. They can be re-enabled in your browser settings.";
-    let buttons = [
-      {
-        label: "Manage Add-Ons",
-        accessKey: "M",
-        callback() {
-          win.BrowserOpenAddonsMgr("addons://list/extension");
-        }
-      },
-    ];
-
-    let nb = win.document.getElementById("high-priority-global-notificationbox");
-    nb.appendNotification(message, "non-mpc-addons-disabled", "",
-                          nb.PRIORITY_WARNING_MEDIUM, buttons);
-  },
-
   _firstWindowTelemetry(aWindow) {
     let scaling = aWindow.devicePixelRatio * 100;
     try {
       Services.telemetry.getHistogramById("DISPLAY_SCALING").add(scaling);
     } catch (ex) {}
   },
 
   // the first browser window has finished initializing
@@ -1047,20 +1023,16 @@ BrowserGlue.prototype = {
           if (addon.signedState <= AddonManager.SIGNEDSTATE_MISSING) {
             this._notifyUnsignedAddonsDisabled();
             break;
           }
         }
       });
     }
 
-    if (AddonManager.nonMpcDisabled) {
-      this._notifyDisabledNonMpc();
-    }
-
     if (AppConstants.MOZ_CRASHREPORTER) {
       UnsubmittedCrashHandler.init();
     }
 
     this._sanitizer.onStartup();
     E10SAccessibilityCheck.onWindowsRestored();
 
     this._scheduleStartupIdleTasks();

--- a/toolkit/mozapps/extensions/AddonManager.jsm
+++ b/toolkit/mozapps/extensions/AddonManager.jsm
@@ -613,17 +613,16 @@ var gUpdateEnabled = true;
 var gAutoUpdateDefault = true;
 var gHotfixID = "";
 var gWebExtensionsMinPlatformVersion = "";
 var gShutdownBarrier = null;
 var gRepoShutdownState = "";
 var gShutdownInProgress = false;
 var gPluginPageListener = null;
 var gBrowserUpdated = null;
-var gNonMpcDisabled = false;
 
 /**
  * This is the real manager, kept here rather than in AddonManager to keep its
  * contents hidden from API users.
  */
 var AddonManagerInternal = {
   managerListeners: new Set(),
   installListeners: new Set(),
@@ -3267,20 +3266,16 @@ this.AddonManagerPrivate = {
      if (!extensionId || typeof extensionId != "string")
        throw Components.Exception("extensionId must be a string",
                                   Cr.NS_ERROR_INVALID_ARG);
 
     return AddonManagerInternal._getProviderByName("XPIProvider")
                                .isTemporaryInstallID(extensionId);
   },
 
-  set nonMpcDisabled(val) {
-    gNonMpcDisabled = val;
-  },
-
   isDBLoaded() {
     let provider = AddonManagerInternal._getProviderByName("XPIProvider");
     return provider ? provider.isDBLoaded : false;
   },
 };
 
 /**
  * This is the public API that UI and developers should be calling. All methods
@@ -3791,20 +3786,16 @@ this.AddonManager = {
   getPreferredIconURL(aAddon, aSize, aWindow = undefined) {
     return AddonManagerInternal.getPreferredIconURL(aAddon, aSize, aWindow);
   },
 
   get webAPI() {
     return AddonManagerInternal.webAPI;
   },
 
-  get nonMpcDisabled() {
-    return gNonMpcDisabled;
-  },
-
   get shutdown() {
     return gShutdownBarrier.client;
   },
 };
 
 this.AddonManager.init();
 
 // load the timestamps module into AddonManagerInternal

--- a/toolkit/mozapps/extensions/internal/XPIProviderUtils.js
+++ b/toolkit/mozapps/extensions/internal/XPIProviderUtils.js
@@ -27,19 +27,16 @@ XPCOMUtils.defineLazyModuleGetter(this, 
 XPCOMUtils.defineLazyModuleGetter(this, "DeferredSave",
                                   "resource://gre/modules/DeferredSave.jsm");
 XPCOMUtils.defineLazyModuleGetter(this, "OS",
                                   "resource://gre/modules/osfile.jsm");
 XPCOMUtils.defineLazyServiceGetter(this, "Blocklist",
                                    "@mozilla.org/extensions/blocklist;1",
                                    Ci.nsIBlocklistService);
 
-XPCOMUtils.defineLazyPreferenceGetter(this, "ALLOW_NON_MPC",
-                                      "extensions.allow-non-mpc-extensions");
-
 Cu.import("resource://gre/modules/Log.jsm");
 const LOGGER_ID = "addons.xpi-utils";
 
 const nsIFile = Components.Constructor("@mozilla.org/file/local;1", "nsIFile",
                                        "initWithPath");
 
 // Create a new logger for use by the Addons XPI Provider Utils
 // (Requires AddonManager.jsm)
@@ -1523,17 +1520,16 @@ this.XPIDatabaseReconcile = {
               if (xpiState.mtime < oldAddon.updateDate) {
                 XPIProvider.setTelemetry(oldAddon.id, "olderFile", {
                   mtime: xpiState.mtime,
                   oldtime: oldAddon.updateDate
                 });
               }
             }
 
-            let wasDisabled = oldAddon.appDisabled;
             let oldPath = oldAddon.path || descriptorToPath(oldAddon.descriptor);
 
             // The add-on has changed if the modification time has changed, if
             // we have an updated manifest for it, or if the schema version has
             // changed.
             //
             // Also reload the metadata for add-ons in the application directory
             // when the application version has changed.
@@ -1551,28 +1547,16 @@ this.XPIDatabaseReconcile = {
               newAddon = this.updateCompatibility(installLocation, oldAddon, xpiState,
                                                   aOldAppVersion, aOldPlatformVersion,
                                                   aSchemaChange);
             } else {
               // No change
               newAddon = oldAddon;
             }
 
-            // If an extension has just become appDisabled and it appears to
-            // be due to the ALLOW_NON_MPC pref, show a notification.  If the
-            // extension is also disabled for some other reason(s), don't
-            // bother with the notification since flipping the pref will leave
-            // the extension disabled.
-            if (!wasDisabled && newAddon.appDisabled &&
-                !ALLOW_NON_MPC && !newAddon.multiprocessCompatible &&
-                (newAddon.blocklistState != Ci.nsIBlocklistService.STATE_BLOCKED) &&
-                newAddon.isPlatformCompatible && newAddon.isCompatible) {
-              AddonManagerPrivate.nonMpcDisabled = true;
-            }
-
             if (newAddon)
               locationAddonMap.set(newAddon.id, newAddon);
           } else {
             // The add-on is in the DB, but not in xpiState (and thus not on disk).
             this.removeMetadata(oldAddon);
           }
         }
       }

--- a/toolkit/mozapps/extensions/test/xpcshell/test_multiprocessCompatible.js
+++ b/toolkit/mozapps/extensions/test/xpcshell/test_multiprocessCompatible.js
@@ -1,11 +1,9 @@
 Components.utils.import("resource://testing-common/httpd.js");
-Components.utils.import("resource://gre/modules/osfile.jsm");
-
 var gServer;
 
 const profileDir = gProfD.clone();
 profileDir.append("extensions");
 
 const NON_MPC_PREF = "extensions.allow-non-mpc-extensions";
 
 Services.prefs.setBoolPref(PREF_EM_CHECK_UPDATE_SECURITY, false);
@@ -191,153 +189,16 @@ add_task(async function test_disable() {
 
   await testOnce(undefined);
   await testOnce(true);
   await testOnce(false);
 
   Services.prefs.clearUserPref(NON_MPC_PREF);
 });
 
-// Test that the nonMpcDisabled flag gets set properly at startup
-// when the allow-non-mpc-extensions pref is flipped.
-add_task(async function test_restart() {
-  const ID = "non-mpc@tests.mozilla.org";
-
-  let xpifile = createTempXPIFile({
-    id: ID,
-    name: "Test Add-on",
-    version: "1.0",
-    bootstrap: true,
-    multiprocessCompatible: false,
-    targetApplications: [{
-      id: "xpcshell@tests.mozilla.org",
-      minVersion: "1",
-      maxVersion: "2"
-    }]
-  });
-
-  Services.prefs.setBoolPref(NON_MPC_PREF, true);
-  let install = await AddonManager.getInstallForFile(xpifile);
-  await promiseCompleteAllInstalls([install]);
-
-  let addon = await AddonManager.getAddonByID(ID);
-  do_check_neq(addon, null);
-  do_check_eq(addon.multiprocessCompatible, false);
-  do_check_eq(addon.appDisabled, false);
-
-  // Simulate a new app version in which the allow-non-mpc-extensions
-  // pref is flipped.
-  await promiseShutdownManager();
-  Services.prefs.setBoolPref(NON_MPC_PREF, false);
-  gAppInfo.version = "1.5";
-  await promiseStartupManager();
-
-  addon = await AddonManager.getAddonByID(ID);
-  do_check_neq(addon, null);
-  do_check_eq(addon.appDisabled, true);
-
-  // The flag we use for startup notification should be true
-  do_check_eq(AddonManager.nonMpcDisabled, true);
-
-  addon.uninstall();
-
-  Services.prefs.clearUserPref(NON_MPC_PREF);
-  AddonManagerPrivate.nonMpcDisabled = false;
-});
-
-// Test that the nonMpcDisabled flag is not set if there are non-mpc
-// extensions that are also disabled for some other reason.
-add_task(async function test_restart2() {
-  const ID1 = "blocked@tests.mozilla.org";
-  let xpi1 = createTempXPIFile({
-    id: ID1,
-    name: "Blocked Add-on",
-    version: "1.0",
-    bootstrap: true,
-    multiprocessCompatible: false,
-    targetApplications: [{
-      id: "xpcshell@tests.mozilla.org",
-      minVersion: "1",
-      maxVersion: "2"
-    }]
-  });
-
-  const ID2 = "incompatible@tests.mozilla.org";
-  let xpi2 = createTempXPIFile({
-    id: ID2,
-    name: "Incompatible Add-on",
-    version: "1.0",
-    bootstrap: true,
-    multiprocessCompatible: false,
-    targetApplications: [{
-      id: "xpcshell@tests.mozilla.org",
-      minVersion: "1",
-      maxVersion: "1.5"
-    }]
-  });
-
-  const BLOCKLIST = `<?xml version="1.0"?>
-  <blocklist xmlns="http://www.mozilla.org/2006/addons-blocklist" lastupdate="1396046918000">
-  <emItems>
-  <emItem  blockID="i454" id="${ID1}">
-  <versionRange  minVersion="0" maxVersion="*" severity="3"/>
-  </emItem>
-  </emItems>
-  </blocklist>`;
-
-
-  Services.prefs.setBoolPref(NON_MPC_PREF, true);
-  let install1 = await AddonManager.getInstallForFile(xpi1);
-  let install2 = await AddonManager.getInstallForFile(xpi2);
-  await promiseCompleteAllInstalls([install1, install2]);
-
-  let [addon1, addon2] = await AddonManager.getAddonsByIDs([ID1, ID2]);
-  do_check_neq(addon1, null);
-  do_check_eq(addon1.multiprocessCompatible, false);
-  do_check_eq(addon1.appDisabled, false);
-  do_check_neq(addon2, null);
-  do_check_eq(addon2.multiprocessCompatible, false);
-  do_check_eq(addon2.appDisabled, false);
-
-  await promiseShutdownManager();
-
-  Services.prefs.setBoolPref(NON_MPC_PREF, false);
-  gAppInfo.version = "2";
-
-  // Simulate including a new blocklist with the new version by
-  // flipping the pref below which causes the blocklist to be re-read.
-  let blocklistPath = OS.Path.join(OS.Constants.Path.profileDir, "blocklist.xml");
-  await OS.File.writeAtomic(blocklistPath, BLOCKLIST);
-  let BLOCKLIST_PREF = "extensions.blocklist.enabled";
-  Services.prefs.setBoolPref(BLOCKLIST_PREF, false);
-  Services.prefs.setBoolPref(BLOCKLIST_PREF, true);
-
-  await promiseStartupManager();
-
-  // When we restart, one of the test addons should be blocklisted, and
-  // one is incompatible.  Both are MPC=false but that should not trigger
-  // the startup notification since flipping allow-non-mpc-extensions
-  // won't re-enable either extension.
-  const {STATE_BLOCKED} = Components.interfaces.nsIBlocklistService;
-  [addon1, addon2] = await AddonManager.getAddonsByIDs([ID1, ID2]);
-  do_check_neq(addon1, null);
-  do_check_eq(addon1.appDisabled, true);
-  do_check_eq(addon1.blocklistState, STATE_BLOCKED);
-  do_check_neq(addon2, null);
-  do_check_eq(addon2.appDisabled, true);
-  do_check_eq(addon2.isCompatible, false);
-
-  do_check_eq(AddonManager.nonMpcDisabled, false);
-
-  addon1.uninstall();
-  addon2.uninstall();
-
-  Services.prefs.clearUserPref(NON_MPC_PREF);
-});
-
 function run_test() {
   createAppInfo("xpcshell@tests.mozilla.org", "XPCShell", "1", "1");
   startupManager();
 
   // Create and configure the HTTP server.
   gServer = new HttpServer();
   gServer.registerDirectory("/data/", gTmpD);
   gServer.start(-1);