I was using Firefox as a back-up for some time. When Opera had a major page incompatibility, I would just go to FF view. Firefox is not a bad browser, but there are a couple of nitpicks I have that prevent me from using it on a permenant basis: Bloated interface, as opposed to utilizing the system's native API; Gecko is not as accurate as WebKit. Firefox memory usage can go into an excess of some 150 MB at times, and my system is only running on 512 MB RAM.
Though it is not necessary a problem with Firefox itself, Firefox's popularity also contributes to some of its security risks. You will notice that some sites detect Firefox, and attempt to override some of your extensions. (such as Ad-Block) This has happened to me on a few ocassions.