FF for many years on Win XP, and then on Win7pro. Then it seemed to be constantly coming with more updates, and slower page loads. I looked around, and decided to give Opera a go. It's less feature-laden than FF, but more than adequate, and will of course import your bookmarks, etc. It requires less mem than FF, and runs quite a bit faster, on my w7 system.
Liked FF, and I kept it up to date, and run Anti-V and super anti-spyware every week, but it just seemed to have got too big for an older Dell laptop.