see http://kb.mozillazine.org/Unable_to_search_or_add_new_engines '''Rename''' or delete the '''search.sqlite''' file in your profile, it will have little consequence as it will be rebuilt from your search.plugins folder also in your profile. You can open your profile folder through the system using '''about:support''' typed into the location bar, then "Open Containing Folder". Renaming or deleting items from the profile should be done with Firefox down (exit throught the File menu or "Firefox" button). If doesn't work, then also rename or delete both the search.sqlite and the search.json file, this will restore all search engines and they will be alphabetical order, you can use the Manage Engines at the bottom of the Search engines drop-down to rearrange. After problem is fixed you can delete the renamed files to cleanup, if you wish.