Reduce overhead of XUL updates [M1354564 et al]

[classilla]

There is now a way to help batch XUL updates in the tab browser and menu code.

https://bugzilla.mozilla.org/show_bug.cgi?id=1354564
https://hg.mozilla.org/mozilla-central/rev/56b70ce1a2ab

It would be nice to see if something similar existed for context menus, too. Those are pain points.

[classilla]

The remaining commit doesn't actually apply to OS X, so I just commented the function out entirely, since it's a waste of memory and processing time.

[classilla]

Not much left to do here.