Firefox's UI has always been the most customizable of any of the major browsers. You will be able to add it back in if it is taken out by default in the future. (Disclosure: I work for Mozilla but not on the UX team.)
I do not believe that, from my experience Opera is much more configurable, at least my current Opera setup suggests that. I have added buttons to my UI that I would have to install addons for in Firefox.
