3 times since I upgraded to Firefox 42.0 (November 4) it has slowed down by a factor of, perhaps, millions: I would move the mouse as if to highlight some text but only 10 minutes later would it highlight; I tried to open a new tab but it wouldn't get around to opening that page it opens for new tabs for hours.

The second time I started top and saw that Firefox was using 100% or nearly 100% of CPU time (though this didn't slow down other apps) and its RAM use would increase .1% nearly every refresh. After a few hours it would exhaust RAM, quit, reporting a failure to allocate 0x100 bytes and a segmentation fault.

I ran Opera to use the Internet. Cor-el @ Mozilla had nothing useful to suggest.

I'm not looking for a diagnosis, I just wanted to know if this is a memory leak.

what are your History settings in Preferences => Privacy?

If it kept growing in memory use and didn't release it then it's possibly a memory leak and if it didn't then it isn't. That's kind of the definition of a memory leak.

You could open an instance of top in a terminal to observe Firefox's use of memory over time. That might tell you something.

I'd say textbook definition of memory leak, combined with something running wild on the CPU. Did you install a fishy add-in?