I would suggest looking up the words in that language for the language setting. At least that's how I would fix that problem I have done it before and it took me hours to figure that solution out.

Agreed. In this case before I posted I did search for "Language Settings" within Firefox, found it, and simply set it back to English. Then I continued to post the question because it wasn't about "how do I change the language setting back to English" but rather "how do these accidents occur in first place". This is also why I pre-emptively wrote "Human Error" because I have to imagine that the language shift on my Mac laptop (which nobody uses except me) was not a result of some bizarre bug in Firefox... But rather I am trying to imagine if some accidental pressing of Command/ Control/ Shift / Option along with some letter of the alphabet triggered some language setting shortcut? I don't really need a "solution" here, because it was easy to reset my language to English. But if somebody knew that YES there is a keyboard shortcut combo that can activate a language setting change, that would help to know that. I am a poor typist and only use hunt-and-peck so it is common for me to accidentally press modifier keys while i am pecking along :)