If Firefox is crashing there's probably a fault somewhere within the software.
One of the great things about Firefox is that it's easy to repair. Simply run the installation program again. (If you downloaded Firefox you've probably still got the installation program on your PC. Otherwise simply download it again). You don't need to uninstall Firefox before doing so. Indeed, it's better that you don't, because re-installing Firefox 'on top of itself' will preserve all of your bookmarks and settings.
Once you've reinstalled Firefox you should find that everything is back to normal.
Chris