In Firefox go to Tools > Options > Advanced > General and click 'Check now'.
If Firefox is no longer set as your default browser (which can happen if registry data becomes corrupt) click 'Yes'
If Firefox is (apparently) your default browser, see the 'Windows XP' section here to set it again via the control panel:
http://support.mozill...browser-does-not-work
If that doesn't work, trying setting Internet Explorer as your default browser, restarting your PC and then setting Firefox as your browser again.
If that doesn't work, try using System Restore to set your registry back to a date before the problem arose. (Start > All Programs > Accessories > System Tools > System Restore).
If that still doesn't work, try an 'on top' installation of Firefox. (i.e. reinstall it WITHOUT first uninstalling it).
If that doesn't work, try uninstalling Firefox and doing a 'clean install'.
If that doesn't work, open a bottle of something alcoholic!
Chris