The best thing you can do is just reset Firefox to defaults. To do that please follow the steps given below:
1. Open Firefox and click on HELP
2. Click on TROUBLESHOOTING INFORMATION
3. Click on the RESET FIREFOX on the upper right side of the page.
4. Confirm Reset and click on Finish
You are good to go!
Resetting Firefox will take it back to company defaults and will be as good as new.
Note: This utility will delete all the bookmarks, cookies, add-ons, form autofill data, saved passwords, toolbars, browsing history, site preferences etc.
You will have to update Firefox again after resetting it. You will need to re-install all the add-ons including Flashplayer, Java, etc.