I've been using Firefox for years. I have a couple of niggles with it, and I've tried IE9/Opera 11.x/Chrome but I keep coming back to FF.
I think browser choice is largely down to UI preference these days, and unfortunately everyone has decided to copy Chrome (perhaps with a dash of the Opera menu). One thing I like about FF is that I can set it back to (pretty much) how it looked in <=FF3. I'm used to things a certain way, I tried the 'improved' version but I prefer it the way it was. <shrugs>
NoScript is also a huge reason why I stay with FF. Opera has similar-ish features but I've never been able to make them work as well as NoScript works for me.
@ Greenman
Steltek is probably right.
You can start FF's profile manager by running Firefox from the command line and putting " -p" on the end. Create another profile, try it in that, if it works, you know that it's a profile issue. Stuff like sign-in info and bookmarks/history can be pulled across to the new profile.