Shortly after upgrading to Firefox 3.6 on the Mac, I noticed a lot of Flash elements offset by one pixel on the top and left. I thought I was going crazy, until I downgraded to 3.5.x and it went away. I tried to find a workaround or explanation, but there was none. Like I said, minor yet irritating.
Apparently, it crops up if the elements are centered and if the browser window is sized to an even width. Thankfully, Mozilla now recognizes the bug, it's become a priority and is working on a fix. (Note: this is not an issue with 'focus' or 'outline' CSS properties.)
Indeed, this is a minor issue, but if you have any Flash elements that are width or height sensitive, chances are they're affected to this bug. I posted this note to save any of you Mac Firefox folks out there the headache of tracking down this issue.