This could be caused by a corrupt file which stores information about your toolbar configuration. Although it is a text file, its internal structure is somewhat inscrutable, so it's easiest to have Firefox regenerate it for you. This will reset all of your toolbar customizations (e.g., icon positions) back to their defaults.

Open Help > Troubleshooting Information and click the Show Folder button to open your settings folder; keep that window open for a minute and return to Firefox

Completely exit Firefox

In the settings folder window, rename localstore.rdf to localstore.old (then you can close this window)

Restart Firefox

Your default set of search plugins should be restored to the search box.

Any luck?

If it didn't work, can you confirm that you have a google.xml file in this folder?

C:\Program Files (x86)\Mozilla Firefox\searchplugins

Try this extension:

• https://addons.mozilla.org/firefox/addon/searchreset/

See also;

• http://kb.mozillazine.org/Unable_to_search_or_add_new_engines

If the searchplugins folder is empty then you need to reinstall Firefox.
It will probably work if you reinstall Firefox on top of the current installation, but if not then you need the delete the Firefox program folder.

Download a fresh Firefox copy and save the file to the desktop.

• Firefox 13.0.x: http://www.mozilla.org/en-US/firefox/all.html

Your bookmarks and other profile data are stored elsewhere in the Firefox Profile Folder and won't be affected by a reinstall, but make sure that you do not select to remove personal data if you uninstall Firefox.

• http://kb.mozillazine.org/Profile_folder_-_Firefox
• http://kb.mozillazine.org/Profile_backup

• http://kb.mozillazine.org/Standard_diagnostic_-_Firefox#Clean_reinstall

I received two solutions to my problem and decided to give the easiest one a try first, so I installed a new version of Firefox over the old one and that solved the problem.