You put GTK apps, and only show firefox - is it gtk apps, or is it firefox which has it's own font settings. I'd expect them to look different to your plasma settings by default. So basically I have two apps with different font settings which seems pretty normal.

I've shown Firefox as a pure example, since for a webpage I can provide a meaningful comparison between browsers.
Here's Software Center

I had the same issue on Arch Linux after update recently. I was able to fix it by disabling then enabling anti-aliasing and rebooting.