I noticed that too -- I'm looking into it this morning.
Florian, what OS are you on? When did you take the mdn-2013.png screenshot?
Ubuntu 13.10 and the screenshot was taken during the break for a blogpost. So the regression is probably from the last few days this year...
Commits pushed to master at https://github.com/mozilla/kuma https://github.com/mozilla/kuma/commit/b90408585799e84782359bfd0f254d8876ff507e fix bug 958487 - Ensure tabzilla fonts don't mess with our design https://github.com/mozilla/kuma/commit/10d0ea67dcc5ba09fad7a85c0a34c5029433b23c Merge pull request #1868 from darkwing/bold-958487 fix bug 958487 - Ensure tabzilla fonts don't mess with our design