There's been some problems with Firefox, especially version 2.0.0.8, where it is reported to be crashing. If you are using that version, start Firefox (Start > Al Programs > Mozilla Firefox > Firefox (Safe Mode)) in Safe Mode and update it (Help > Check for Updates).
If it's still unstable... then it might be some incompatible addons or a corrupted profile.
http://www.mozilla.org/support/firefox/profile#newFor incompatible addons, start Firefox in Safe Mode again and slowly enable them one by one till you find the problematic one. Remove that one and search the Addons database for a similar one or updates for it.