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.

Thanks, but the only scanner listed in that article is Malware Bytes, which is not picking up any problems on my computer. Following the links on that page takes me around in a circle back to that page.

PabloSkilz said

Thanks, but the only scanner listed in that article is Malware Bytes,

There are five scanners listed in that article though .....

Especially for Mac : https://www.malwarebytes.com/mac/

Other Mac users with this problem had a rogue extension named "Search Updater".

You can view, disable, and often remove unwanted or unknown extensions on the Add-ons page. Either:

• Command+Shift+a (Windows: Ctrl+Shift+a)
• "3-bar" menu button (or Tools menu) > Add-ons
• type or paste about:addons in the address bar and press Enter/Return

In the left column of the Add-ons page, click Extensions.

Then cast a critical eye over the list on the right side. Any extensions Firefox installs for built-in features are hidden from this page, so everything listed here is your choice (and your responsibility) to manage. Please disable or remove Search Updater. If in doubt about any of the others try disabling it.

After that, you should regain control of your settings. You then can restore your preferred default. See: Change your default search settings in Firefox.

Success?

jscher, I owe you beers at least. I'd gone through the list of extensions several times with a suspicious eye (since given all the indicators this was a likely source of the problem) and looked at Search Updater on each occasion apparently without noticing it. Now it's gone and the problem is solved. I am impressed by your search skills. I went through several forums and different search criteria, but only found one reference to this problem, which was unanswered. Now I should try to find that reference and provide this advice. Incidentally, the default search engine as named in the relevant section did not need to be altered: It always read Google throughout the hijack period.

Hi PabloSkilz, thank you for reporting back. I think it's a new issue, it's just come up a few times in recent days. I haven't seen any explanation of how it spreads.