Have a dual-display setup on a MacBook Pro 15" running El Capitan. (Although I observed this in Yosemite also.) I have a 1920x1080 display "on top" of my Mac's built-in laptop display. When I open a new window (either the browser or a new message window) in the "top" (1920x1080) display I get a tiny window about 1/2 of normal size rendered on a big, normal size canvas. (See attachment.) I could not find a bug report for this. I can get the window to render properly by dragging it back "down" to the other display (the laptop display). I can also close the malformed browser window in the top display, click on Thunderbird in my menu bar, and it will reopen and properly render in the top display. Anybody seen this? Thanks -wsanders