There are lots of reasons why Firefox is running slow - lots of extensions to load, other progs running and so on but I wont bother with that for now because Firefox 3.5 final will be released in the next few days.
I suggest you uninstall your current version (which I'm assuming is 3.0.*) and install this
Firefox 3.5 RC3 version. It is more than likely the last release candidate. When the final version is released, you will be prompted to download it.
If you need a specific language version you can find it
here.
Let us know how you get on and whether it has resolved your problems.
btw - Firefox 3.5 is a lot faster than 3.0.*