Argh! It came back! Went to my fave web page again today and the fonts in the textarea were screwed up again. So now I'm really baffled.
More digging... FF *does* give a way to see the fontname actually selected! The dev inspector has a font tab hidden in a dropdown arrow. It'll show the actual font selected for a single html element. Just what we need.
So... for the textarea in question, it shows something interesting:
"" Liberation Sans
Lohit Bengali
Lohit Bengali ""
I have no idea why it shows 3 fonts for 1 element. But "Bengali" and "Lohit" seem out of place. (Liberation is the one I manually selected as the FF default in previous steps.)
So: dnf remove lohit*
Restart FF, and boom, problem solved (again?). The problem appears to be the mixing of different fonts in a single element, which also explains why the numbers in some cases looked like a different font.
The FF font tab now shows just Liberation, which is as it should be.
I still don't know why this was unique to my main FF profile (as --private to firejail also solved it), nor why FF wants 3 fonts for 1 element, nor why it's magically picking lohit even though that's not specified anywhere (settings, html css). Also doesn't explain why it broke mid march when almost nothing but FF has been updated since then.
And why is anything Bengali being enabled in my profile??
I wonder if there's a chance there's some master Fedora font-choosing file / system that suddenly had "Lohit" appear (higher) in its list of sans-substitutes?