I have Firefox on two computer (plus my phone), both of them are running Linux Mint. On pc#1 Firefox behaves ok; on pc#2 instead it's abnormally zoomed in and it is very annoying to use. I am logged in with my Mozilla account on both devices, they have the same settings and Firefox is the only application displaying this behavior. If it matters, pc#1 has a bigger screen. I've attached a screenshot of the same page visualized on both devices as to show the difference.

(I am posting this from pc#1)