bug 1397562 - Disable update staging on OS X and Linux via the app.update.staging.enabled preference. r=mhowell

There are a couple of bugs that indicate instability with update staging on OS X and Linux so disable it especially since it is already disabled on Windows

browser/app/profile/firefox.js

@@ -162,12 +162,8 @@ pref("app.update.silent", false);
 // app.update.badgeWaitTime is in branding section
 
 // If set to true, the Update Service will apply updates in the background
-// when it finishes downloading them.
-#ifdef XP_WIN
+// when it finishes downloading them. Disabled in bug 1397562.
 pref("app.update.staging.enabled", false);
-#else
-pref("app.update.staging.enabled", true);
-#endif
 
 // Update service URL:
 pref("app.update.url", "https://aus5.mozilla.org/update/6/%PRODUCT%/%VERSION%/%BUILD_ID%/%BUILD_TARGET%/%LOCALE%/%CHANNEL%/%OS_VERSION%/%SYSTEM_CAPABILITIES%/%DISTRIBUTION%/%DISTRIBUTION_VERSION%/update.xml");