The search.URL is what is used for location bar searches
Verify that you are looking at the Firefox search bar
click on the favicon at the left of the search bar, do you see your search engines,
if so click on the one you want.
If not at the bottom is "Manage Search Engines", you can click on that for the default set of about 5 search engines.
If you had a lot more then the original set delete your search.sqlite and your search.json files in your profile with Firefox down, which should bring back all of your installed engines in alphabetical order from the searchplugins folder in your profile. Use the "Manage Search Engines: to rearrange their order.