If you think about it, the forward button is disabled about 80% of the time -- so it doesn't really need to always be there. The forward button isn't being removed completely; it'll just be hidden when it's not clickable. It will slide out when there's a "forward" URL available.
I can usually mouse to within a few dozen pixels of my target without looking at the screen, thanks to a combination of muscle memory and spatial sense.
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.