Creating as UNCONFIRMED because I didn't reproduce this issue myself - simply reposting a report from an Adblock Plus user. It seems that on OS X the add-on bar is displayed on top of the window resizer - with the result that the Adblock Plus icon for example covers up almost all of the resizer.

I thought it removed the windows resizer completely rather than covered it up?

I don't have a Mac to test this. The original report said literally: "the adblock icon in the new Add-on bar is covering up a large part of where you click to grab and resize the browser window."

Dupe of bug 489303?

I thought that bug 489303 was Windows-only - but looking closely (also at the patch attached), it should resolve the Mac issue as well. Yes, this is a dupe.