Assuming it's Firefox 3.... First thing to do is isolate if it's the App or your personal settings, so if there is another account on your Mac try logging into that first and seeing what the effect is. If you don't, you can try the following anyway.
Presuming you have another account to log in to - If it's the same problem, download Firefox again and overwrite the existing one in /Applications.
If that works ok, go on to the next step to troubleshoot the Firefox settings on your account.
Try renaming the org.mozilla.firefox.plist, which you'll find in /Users/xxyourusernamexx/Library/Preferences.
If that doesn't work, try this...
Find the the following location: /Users/xxyourusernamexx/Library/Application Support/Firefox and rename the Firefox folder (don't delete it - you may want to put it back. I usually stick "old" in front so it's easy to find again).
This removes your Firefox profile, so next time you launch it'll be missing all your bookmarks and stuff - you can always delete the new Firefox folder that gets created and rename the "oldFirefox" folder you originally renamed back to "Firefox" if it doesn't fix it.
Hope this helps.