Firefox, like Chrome, automatically updates itself (I'm not sure that can be turned off any more),

You can.

Menu, options, advanced, update. Done.

Weirdly, looking at my add-ons, the one I thought would not be updated (compassmenu) was, but the one I thought was sure to update (colorfultabs) wasn't. And if close tabs to the left isn't updated, I'm not updating either.