Bug 1249608 - Don't run UI migration steps with new profiles. r=gijs

browser/components/nsBrowserGlue.js

@@ -1808,10 +1808,16 @@ BrowserGlue.prototype = {
   _migrateUI: function BG__migrateUI() {
     const UI_VERSION = 36;
     const BROWSER_DOCURL = "chrome://browser/content/browser.xul";
-    let currentUIVersion = 0;
-    try {
+
+    let currentUIVersion;
+    if (Services.prefs.prefHasUserValue("browser.migration.version") {
       currentUIVersion = Services.prefs.getIntPref("browser.migration.version");
-    } catch(ex) {}
+    } else {
+      // This is a new profile, nothing to migrate.
+      Services.prefs.setIntPref("browser.migration.version", UI_VERSION);
+      return;
+    }
+
     if (currentUIVersion >= UI_VERSION)
       return;