Firefox users are complaining about font issues, it seems that the recently released Firefox 17 introduced a bug that affects Windows users who have disabled ClearType.
The problem is that Firefox 17 ignores this setting and applies subpixel anti-aliasing to the fonts, despite users explicitly disabling it.
The issue has been traced back to the new Azure rendering engine, which is now used by default in Firefox. Disabling Azure fixes the issue, but it's not the best of solutions.
Mozilla has fixed the problem, the issue now is whether to release a Firefox 17.0.1 or wait until Firefox 18 is set to go live.
While there is a working patch, Mozilla is weary of releasing it without proper testing. Likewise, disabling Azure altogether would affect all users and is considered the last resort.
The problem is the number one support issue in Firefox 17, so a lot of people are affected. The only fix for now is to disable Azure for content.
Visit about:config find gfx.content.azure.enabled and set it to False if this issue is affecting you.