The greasemonkey logo image for the Add-ons button breaks the window layout on Mac version of Firefox, as can be seen on picture below:

http://twitpic.com/3zxqwd/full

Greasemonkey 0.9.1 @ Firefox 3.6.15pre

Mozilla/5.0 (Macintosh; U; Intel Mac OS X 10.6; en-US; rv:1.9.2.15pre)
Gecko/20110201 Firefox/3.6.15pre (Mac Community Build, ElFurbe)