The search function is no longer working? Hitting return or clicking on SEARCH does nothing.
Are you referring to the Search box on the about:home homepage>
What is the value of the browser.search.defaultenginename on the about:config page?
That should be Google.
Try to reset some preferences to the default with the SearchReset extension:
Note that the SearchReset extension only runs once and then uninstalls automatically, so it won't show on the "Firefox > Add-ons" page (about:addons).
If you do not keep changes after a restart then see: