Although my FF has a google search frame on the far right of the same bar as the url frame. Do you have that?
The Firefox start page comes up by clicking on the small "home" icon on the left. Yes Google is still available by clicking on the magnifying glass. Google used to already be on the start page but has disappeared and I get the error message. This is the url that comes on my start page.
You might as well go through and review every nick and cranny of your options.
I have been assuming you put your computer through all of the clean up and empty procedures, clear cache, clear cookies, defrag, run CCleaner, run Malawarebytes, run full scan of your anti-virus software.........
I try reading the FF help pages too. But usually, folks here say things more clearly than FF does.