Well I've solved part of the problem. I did find a list of key shortcuts for Ff, but it turns out
not to be a key shortcut. My right-click menu, in Firefox (and not in IE) offers an option called "Check Spelling"!

Who knew?!! But that option is only there if the mouse pointer is in a text field. I use that right-click so seldom because I've gotten into the habit of using Ctrl + C or V, etc. for copy/paste, etc. In IE, all there is usually, is cut, copy, paste and delete. I've only been using Firefox for a year or 2, and never really paid attention to the right-click menu, since I use it so infrequently.
So how did I manage to click it, you ask? I actually have very poor mouse dexterity, and am not surprised at all, that I clicked it accidentally,

Actually I probably right-clicked on purpose, but didn't intend to choose that option. Probably my finger slipped when I was trying to right-click outside the text field. Then I probably had to click again to get rid of that menu, but clicked Check Spelling instead
At this moment, I have it narrowed to Firefox, but I'm still trying to learn if it's related to some add-on, plug-in or extension. Does anyone else who uses Firefox have such an option? Maybe all Ff users have it?? I actually have a topic in the FF forums, but it can't hurt to ask here. I don't think any potentially conflicting info would be a problem, in this case (although usually I wouldn't post topics in more than one forum).
Anyway, mystery partly solved
