Hi fuzzymazoid, I updated a Windows 10 system to Firefox 50 and didn't notice any new gap. But it might be a couple months behind on Windows updates so I'll need to check that later.

Meanwhile, have you ever created or edited a userChrome.css file which could contain rules modifying Firefox's user interface? (http://kb.mozillazine.org/UserChrome.css)

Hi jscher2000, thanks for your reply!

I do not have a userChrome.css file (nor do I have the "chrome" folder) in my profile.

I found this weekend that the problem is only on my external monitor. Here's my setup:

HP laptop @ 3840 x 2160 with 250% zoom (recommended/default settings)

ViewSonic 24" 4k monitor @ 3840 x 2160 with 200% zoom (recommended/default settings)

Windows is set to extend the desktop between the monitors.

Opening Firefox on my laptop screen, I do not have a gap between the browser tabs and the top of the monitor -- it works as I remember.

Moving Firefox to the ViewSonic monitor and maximizing, the gap exists, and clicking at the very top of the screen does nothing.

I've had this same monitor setup at the office for about 5 months, and to the best of my memory this didn't become a problem until the update to Firefox 50 a couple of days ago.

I don't have any personal experience with "higher than full HD" resolution, and I haven't extended by desktop for at least 12 months, so... I'm just going to throw out some questions.

• Is there any difference in the height of the Windows Task Bar in the second monitor that could account for the extra pixels?

• Does it make any difference if you click the "restore" button to switch to a resizable window and then maximize again?

• If you launch a new window from the window on the second monitor, does it open there with the tabs correctly aligned to the top?

I might have figured it out. First I use Firefox on the laptop by itself -- no external monitor -- and then put the machine to sleep. Then, I plug in the external monitor and wake the machine up, the problem exists. Closing and reopening Firefox doesn't resolve the problem. It only goes away if I reboot (or perhaps sign out of Windows, but haven't tried that yet).

My wild, uneducated guess is it has something to do with the different zoom settings between the two monitors. Making the switch to 4K means everything has to zoom to be usable. I've found many of my programs have a hard time with zoom -- I frequently get toolbar buttons that are ridiculously tiny... sometimes coupled with exceptionally large fonts (a great combination). Lots of smaller developers just never tested their programs with zoom.

jscher2000 -- do you think I should report this to Mozilla as a bug? It's an obscure bug, to be sure.

For my immediate needs, I'm just going to make the zoom level the same on both monitors and see if that solves the problem.

If you use the same resolution, for example, 1920 x 1080 (full HD) on all displays, that should avoid the problem, but it's not always an attractive solution. You might want to submit a bug, they will need someone with high resolution monitor to test possible fixed.

https://bugzilla.mozilla.org/enter_bug.cgi

Jut a comment that I am not seeing this on Windows 10.

I do a have an area above the tab as in your screenshot but I am unable to select and click that point it is presumably too narrow.

Thanks all!

I don't think it's the resolution, as the resolution is the same on both monitors. I think it's the difference in zoom level Windows recommends. The physically smaller monitor (laptop) defaults to 250% zoom while the physically bigger external monitor defaults to 200% zoom. Whether it's a Firefox problem or a Windows problem (or a video card driver problem, etc), something is confused when you try to use both.

I'll submit to Mozilla when I have time and see if anyone else can duplicate. If it's just my computer, then I plan to go back to a 40-lb 800x600 CRT.