Mozilla released an update for the desktop version of Firefox 17 on November 30th, 2012 with Firefox 17.0.1 to address these two issues:

• 17.0.1: Reverted user agent change causing some website incompatibilities
• 17.0.1: Font rendering issue in Firefox 17.0 bug 814101

Users should be prompted to update soon, or can manually check for update via Firefox Button > Help > About Firefox or download and install via the getfirefox.com site. Note: At this time there is not an update to Thunderbird 17.0.1.