After I played some ol' skool games like Age of Empires I, Google turned up with bold font in Firefox (any version). I'm running Windows 7 x64, and application restart or system reboot isn't helping.
How do I fix it?
The problem is often caused by hardware acceleration, which causes the application to render some fonts in bold. It can also be the result of compatibility problems with the current operating system.
I found two possible solutions:
Both possible solutions are described here.
Disabling hardware acceleration solved my problem.