Type about:customizing<enter> in the address bar. Look for Show / Hide Toolbars at the bottom left.

There is also a Restore Defaults button at the bottom middle.

Type about:preferences#search<enter> in the address bar. Select what you want as your default search engine here.

Try to delete the search.json file and possible search-metadata.json and search.sqlite files in the Firefox profile folder to reset the search engines to the default.

You can use this button to go to the current Firefox profile folder:

• Help > Troubleshooting Information > Profile Directory: Show Folder (Linux: Open Directory; Mac: Show in Finder)
• http://kb.mozillazine.org/Profile_folder_-_Firefox

Firefox will rebuild the search.json file from the default built-in search engines that come with Firefox and the searchplugins folder in the Firefox profile folder.