Try restarting in safe mode in about:support to see if this still persists. To make sure it's being changed properly, that should be changed in about:preferences#search . As long as you set that preference and continue using the same profile, which can be seen in about:profiles, the search engine should not switch. Same with your home page.

Type about:addons<enter> in the address bar to open your Add-ons Manager. Hot key; (<Control> [Mac=<Command>] <Shift> A)

In the Add-ons Manager, on the left, select Extensions. Remove “ yahoo “ Anything.

Also remove SafeSearch, SearchAssist Incognito, FileConverter Addon, SearchLock, DiscreteSearch, WebSearch, Findwide Search Engine, securedsearch

Now do the same in the computer’s Programs Folder.

Windows: Start > Control Panel > Uninstall Programs. Mac: Open the Applications folder. Linux: Check your user manual.

I did not find anything. I have no add-ons or extensions. I did have a file SafeSearch that I removed months back. It is still happening.

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 malware scanners. Each works differently. If one program misses something, another may pick it up.

FredMcD...

Found the AdWare and it is fixed. Thanks

Rusty

Glad to help. Safe Surfing.