Shut down and rebooted my machine to make sure it's not my computer; problem persists.

Works for me (Firefox 119.0b3 on Windows 10):

Can you check which font is being used in Firefox developer tools?

Does it work correctly when you're logged out?

Log in-status makes no difference. font used is Arial.

I did some more digging and found this happened at the same time that firefox updated to 118.0.1
Looking at the release notes, I find "The visibility of fonts to websites has been restricted to system fonts and language pack fonts to mitigate font fingerprinting in Private Browsing windows"

Could that be the answer? (and maybe they changed it back in 119)

Maybe? I have no idea how that feature works. Maybe it's enabled in non-private windows too depending on your settings. I also saw another complaint recently about fonts used by Firefox changing: T347522#9210020 (although this person had the issue in 119 and not 118, so it may be a complete coincidence).

I would suggest reporting the problem at https://bugzilla.mozilla.org/, either it's a bug or they will be better able to figure out why it happens.

My thought exactly. I reported this to the Firefox developers; it does not seem to be anything wikimedia can do about it. Let's commit this task to the archives for future reference. Thanks.