Bug 1384541 - Add a UI migration from compact themes to compact mode. r=dao

MozReview-Commit-ID: A6kr1QM5Nxu

--HG--
extra : rebase_source : c7065dc43723a3e086238ebaaa5b2413f75f9b53

browser/components/nsBrowserGlue.js

@@ -1735,7 +1735,7 @@ BrowserGlue.prototype = {
 
   // eslint-disable-next-line complexity
   _migrateUI: function BG__migrateUI() {
-    const UI_VERSION = 50;
+    const UI_VERSION = 51;
     const BROWSER_DOCURL = "chrome://browser/content/browser.xul";
 
     let currentUIVersion;
@@ -2095,6 +2095,16 @@ BrowserGlue.prototype = {
       }
     }
 
+    if (currentUIVersion < 51) {
+      // Switch to compact UI density if the user is using a formerly compact
+      // dark or light theme.
+      let currentTheme = Services.prefs.getCharPref("lightweightThemes.selectedThemeID", "");
+      if (currentTheme == "firefox-compact-dark@mozilla.org" ||
+          currentTheme == "firefox-compact-light@mozilla.org") {
+        Services.prefs.setIntPref("browser.uidensity", 1);
+      }
+    }
+
     // Update the migration version.
     Services.prefs.setIntPref("browser.migration.version", UI_VERSION);
   },