So I'm struggling a bit with the Firefox transition, and I made the mistake of doing a refresh (deletes all extensions and settings and resets everything back to a basic Firefox install). This caused me more problems, but the most baffling one is that everything about the browser is 50% larger now. Webpages now look the same at 67% zoom as they used to look at 100%, and all of the menu bars / fonts / tabs / etc are the same way.

I'm suspicious that this might be an issue with scaling in Plasma rather than something specific to Firefox because Chromium has been the same way for a while. (I hardly use Chromium, so I haven't really cared about this until now.) However, those are the only two programs which are doing this. No others. Any ideas?

Using KDE Mint, 4.14.2

Well, something you can try to see if it is related to Plasma's scaling or not is to switch to a non-Plasma session and open FF to see if it looks right (and compare that to other apps like Chromium).

Good call airdrik. Similarly, you can close Firefos and then move ~/.mozilla/firefox to elsewhere.

Well... moving my ~/.mozilla/firefox directory didn't solve it, and everything is just as big in Xfce as it is in Plasma. So I still don't know what the problem is, but at least I know that it isn't Plasma. That was a good call.

I'll keep looking, thanks.