User Agent: Mozilla/5.0 (Windows NT 10.0; Win64; x64; rv:57.0) Gecko/20100101 Firefox/57.0 Build ID: 20171206182557 Steps to reproduce: Opened Firefox in my second monitor with different resolution than my first monitor. Clicked on an add-on like "uBlock Origin" that opens it's on screen data or on menu > help > about Firefox. Actual results: Firefox add-ons doesn't show in a correct resolution when I open firefox in my second monitor. also the update window shows in a big window with large fonts. Firefox also shows a long useless bar on top of the tabbar. Expected results: everything shoul be in it's default and actual size

The addon popup dpi issue is bug 1387340. Duping there. The bar at the top of the second monitor window sounds a bit like bug 1347167 to me. You could watch that one or file a separate bug for the space issue.