Created attachment 8522985 [details] firefox-ui-issue.png User Agent: Mozilla/5.0 (Macintosh; Intel Mac OS X 10.10; rv:33.0) Gecko/20100101 Firefox/33.0 Build ID: 20141030112145 Steps to reproduce: First, I have an external Dell P2815Q monitor at work. The browser functions fine when I'm at full resolution of 3840x2160. However that's straining on the eyes so I dropped down to 3008x1692 in scaled mode. After that, starting the browser is all it takes. Actual results: The window appears and there's a big black bar across the top half of my tabs and above. Expected results: Normal browser window UI function
Huh, that looks very broken... does this reproduce if you restart in Firefox's safe mode (Help > restart with add-ons disabled) ? Checking that will help figure out if this is a graphics or widget issue... (I suspect the former, but data>intuition :-) )
Oh, and does this happen on your main display as well, if you scale that?
If I restart with add-ons disabled, the window displays properly, even at full-screen on the external monitor. My laptop screen is scaled as well, to the highest resolution. If I start on just that, the window displays correctly. If I leave the Firefox window at that size and then connect to the external monitor, it stays okay as well.