” Some users of the Firefox web browser have reported issues with blurry
fonts after updating it to the latest release version Firefox 17.0. The
issue seems to only affect users running the Windows operating system …

“…Note that Thunderbird 17.0 is likely affected by the same bug as well. …”

Temporary solution  ghacks.net/2012/11/24/firefox-17-0-1-to-fix-blurry-font-issue-in-the-browser/