Unable to search or add new engines
From MozillaZine Knowledge Base
Unable to search
The "XPL LinkScanner Firefox 2.6" extension can cause searches to fail [1] or Firefox may crash when using various search engines. Disable or uninstall the extension via "Tools -> Add-ons -> Extensions". For more information, see the "XPL LinkScanner" entry in the Problematic extensions article.
Search Bar no longer works and cannot add new search engines
Firefox 1.5: The problem is most likely a corrupt localstore.rdf file. See this article for a solution.
Firefox 2 and above: If the Search Bar no longer works and you can't add new search engines, the problem may be a corrupt "search.sqlite" file. [2]. Try the following solution:
- Exit Firefox and go to your Firefox profile folder.
- Delete (or rename) the file "search.sqlite".
- Re-start Firefox to see if it solves your problem.