Do a hard reset on your browser and see if that fixes it.
I’m not sure how to do it in Firefox, but you do it like so in Internet Explorer:
Go to “Internet options” (through Tools menu selection on older IE, the gear icon on IE 9 or later).
Click “Advanced” tab.
Click “Reset...” button.
On the next window that appears, check “Delete personal settings” checkbox and click “Reset” button to finish up.
Close out IE and restart it.
Another thing that can cause such weirdness is setting the default Windows text size to something other than 100%, which is possible in Vista and later.
Some webpages and programs really don’t like it when you do that.
The best way to adjust text size without all the strangeness is to use the screen resolution instead of just messing with text sizes.