As I said, I completely reinstalled the OS because I was using so much disk space (probably from all the audio samples I had in GarageBand and Logic...)
I really didn't think I was using that much space, but apparently I was, so I just nuked everything and started over. So I may go back and try FF again, because it did seem to handle memory better overall than Chrome did - I regularly have 20+ tabs open, and after a few days (if I let it go that far), Chrome is eating up 7-8 gigs of memory. I let FF go a few days and it never went above 5, and sometimes went back down into the 4 range...
The only reason I suspected FF is because it was the *only* new app I was running when the reboots started to happen. My iMac is otherwise pretty stable, usually staying up for weeks at a time.
**Update**
I went ahead and did the diagnostics test since I was rebooting anyway, and of course, no issues found...
I'm reinstalling Firefox, and I'm gonna give that a try again, because, like I said, it definitely uses less memory (and manages it better) than Chrome does.