Hmm, that's odd.

Is your Firefox retaining other data, such as bookmarks, history, other changed settings, and theme or toolbar layout changes?

Are you choosing a different default search engine from the built-in list (for example, Bing or DuckDuckGo), or are you using an add-on for the new default search engine?

This feels a little like it could be malware/adware, where they're setting the default to an engine that has been blocked, and Firefox is then resetting to default (as that's all it can do).

Might be worth using something like MalwareBytes to check for malware/adware. I'd be interested to know the names of anything it did find.

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 “ Google “ 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.