What Could Make Firefox Render An Incorrect Border Width?

I have a weird problem in a project I'm working on. Firefox (22) magically eats up 1 pixel of my border. All other browsers are fine. Now, the layout is really complex and I can't

Solution 1:

Turns out this only happens when the font size in Windows 7 is set to something else than 100%.

Reported as a bug in Firefox: https://bugzilla.mozilla.org/show_bug.cgi?id=890383