Type about:preferences#search<enter> in the address bar.

Here's our general support article on changing your default search engine: Change your default search settings in Firefox.

But... add-ons can override your settings, so if Firefox seems to be ignoring your preference, we can also assist you in hunting down and removing problem add-ons.

Note that you should be able to access the search settings by clicking the search engine icon in the search bar and click "Change Search Settings".

Do you still see all the built-in search engine in "Options/Preferences > Search"?

If that isn't the case then 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.