Are you talking about the home page? https://support.mozilla.org/en-US/kb/how-to-set-the-home-page

Or the search engine? Type about:preferences#search<enter> in the address bar. You can select any search engine to be your default from here.

You can also remove any engine from here.

I'm having exactly the same problem. I've gone through all the troubleshooting I can see suggested - reset the hoepage, reset defaults, checked for add-ons and extensions, run virus scans repeatedly, deleted and reinstalled Firefox... And yet it keeps re-appearing, so I can only assume there is some kind of program on my hard drive that keeps restoring it. I've noticed that it will not immediately re-appear if I've reset defaults etc and then opened Firefox, but if I have rebooted my computer then it will re-appear when I open Firefox.

Advice on how to get rid of it would be very gratefully received. I worry that it's doing something more sinister than just hijacking my Firefox homepage.

Many thanks AKG

You may have ad/mal-ware. Further information can be found in this article; https://support.mozilla.org/en-US/kb/troubleshoot-firefox-issues-caused-malware?cache=no

Run most or all of the listed free to use malware scanners. Each works differently. If one program misses something, another may pick it up.

It’s very sad, but many software downloaders/ installers will trick you into installing not only their program, but other programs as well.

You have heard of the fine print in shady contracts, right? Well, some installers you need to look at the itsy bitsy teeny weeny fine print.

You are thinking you are giving the installer permission to install the program you want by using the recommended option. But if you use the Manual Option Instead, you discover all kinds of stuff that you do not even know what it is or what it does.

From now on, everyone needs to Use The Manual Option to put a stop to this.

Note that these programs can also change browser/computer settings.