If you find yourself not being able to add a search engine into the search bar chance are you may have a damaged search.sqlite file. In which case this ca be resolved by:
- Locate your Profile Folder
- Close Firefox
- From within your Profile Folder remove or rename the file search.sqlite
- Restart Firefox and attempt to add the new search engine again
- If you are still unable to add the search engine refer to this mozillaZine KB article for additional troubleshooting steps.