Firefox is doomed because its lost its developer foot hold, anecdotally most devs I know refer to it as a joke and use Chrome instead. Its a browser that relies on recommendation to exist, and it no longer is getting recommendation.
On a technical note each version isn't really getting any better, historical problems have never been addressed, version fragmentation is occurring, on several matters its in direct conflict on the w3c standards with other major players.
The UI is polarising (its the major complaint I actually here next to 'laggy behavior'), the options to change it rely on 3rd party modules which are often bugging and not supported between versions. It's never nailed OSX, it just doesn't 'feel' the same as the rest of the operating system and is slightly jarring at times.
Personally if you take a base install of Opera 11 it is exactly how I want FF to be set up, everything works smoothly, fast, and 'clicky'. I'd recommend any FF user to try the new version of Opera for a few days, then go back to FF and see just how different it feels even though they are aesthetically quite similar.