Firefox now tries to adjust the zoom level according to your Windows settings, as well as better supporting high DPI displays. It’s not going to be to everyone’s liking.
I just upgraded to Firefox 22 on this computer and had to do the following in order to view webpages “correctly”:
(1) In a new tab, type or paste about:config in the address bar and press Enter. Click the button promising to be careful.
(2) In the filter box, type or paste pix and pause while the list is filtered
(3) Double-click layout.css.devPixelsPerPx and change its value to 1.0Â for original sized fonts (like they used to be in FF 21 –), or a larger ratio for larger fonts. For example, 1.25 corresponds to 125% font size in Windows display settings.
More info in this thread: How do I set the default font size to a lower value?