Okay, maybe ... just figured out the problem. I was using the Classic YAbb Theme. Changing to Musiconica makes a huge difference!
Maybe I should ask to have the YAbb theme "fixed" up! To my eyes the blue is much easier to deal with.
Fonts are fine now. Matter of fact, the editing font size appears to be larger than the viewing font. And, I tried with Google-Chrome a few minutes ago and it looks the same as Firefox. So, must be the theme
I don't suggest to set the minimum font size in FF. It buggers up the appearance of many web pages. Nicely, FF does remember the magnification setting for individual pages ... but it seems to need "resetting" from time to time.