What version of Firefox?
Have you done a clean install since the previous major upgrade?
Have you tried disabling all extensions, and then re-enabling them one-by-one, to see if one of them is causing the problem?
The development team recommends that you uninstall previous versions of any Mozilla-based browser before installing the new version. This is an especially good idea across major upgrades, e.g. 0.x to 1.x, 1.x to 2.x, etc. On Windows, use add/remove programs to uninstall the old version before installing the new.
In almost all cases, your bookmarks and preferences will be preserved. If your extensions maintain their own preferences in separate files, it's a good idea to make a backup of your profile directory so that these files are not lost by being overwritten if you have to re-install the extensions. If you have a backup, it's usually a simple matter to re-install the extension, and then copy its configuration file from your backup to the new profile directory.
Updating extensions and/or doing a clean re-install cures most 'weird' problems with Mozilla-based browsers.