Firebug, the extremely popular add-on for Firefox is indispensable when it comes to debugging CSS issues. However, with the last update to Firefox, trying to inspect the CSS rules for an anchor element failed.
Issue: The update to Firefox 2.0.0.12 broke something.
Solution: Download (Windows link) and install Firefox 2.0.0.11 (the previous version) and wait for the update to 2.0.0.13 instead.
As the bug report notes, this bug will also screw up other similar extensions such as Chris Pederick's excellent Web Developer extension.