I did a cursory search, but couldn't find if this bug has been reported before.
On the "related questions" sidebar, questions that happen to have a reputation count of three or more digits is overflowing. For example, the post "How can I tell people to get to the point" has 105 reputation, and displays as the below.
You can see this behavior on this page. I'm currently running Firefox 29 on Windows 8.1.
Perhaps the gray box needs to be widened a bit?