Oh thank god it's not just me. I just woke up and I was startled by the font difference. Things even looked a little zoomed out (or zoomed in in some places), but nope, just a sudden change.
Edit: Actually I'm having text look weird in a couple of other places... But not sure how we could both have the same issue?? I'm on Firefox 94.0.1.
Edit 2: Ah, fortunately a fast fix! This is apparently the source of the issue:
https://github.com/mozilla/contain-facebook/issues/858