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 an engine from here.

Does your default change to something else on its own?

FredMcD said

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 an engine from here.

I tried it several times. With next browser update search settings are reset - default search changes to useless and garbage appears again in the list of search engines (like amazon)

Were you updating the browser or using the full installer?

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 “ ******* “ Anything.

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

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.

https://support.mozilla.org/en-US/kb/how-to-fix-preferences-wont-save

Note: Some software, like Advanced SystemCare with Surfing Protection, can protect files in the Firefox profile folder against changes. If you have such software then check the settings or uninstall this software.