Before uninstalling Firefox (and then reinstalling it) try simply resetting it first. (That assumes, of course, that you can actually open it). To do so, start by going to Tools > Add-ons > Extensions and noting down any extensions that you wish to keep (because you'll need to download them again later). Then go to Help > Troubleshooting Information and click on 'Reset Firefox' (at the top right of the page). Lastly reinstall your extensions.
This link explains what is preserved, and what is lost, by clicking that 'Reset' button:
http://support.mozilla.org/en-US/kb/reset-firefox-easily-fix-most-problems
If that doesn't work, try an 'on top' installation next. (i.e. reinstall Firefox WITHOUT uninstalling it first). That also preserves most of your preferences.
If you get to the stage where you think that you really need to uninstall Firefox (and then reinstall it) you may well find that much of your old data is still preserved but it would be wisest to back-up as much as possible first. (You can back-up your bookmarks via Bookmarks > Show all bookmarks > Import and backup.)