User Agent: Mozilla/5.0 (Windows NT 6.3; rv:42.0) Gecko/20100101 Firefox/42.0 Build ID: 20150719030219 Steps to reproduce: 1) Run Nightly 42.0a1 (20150719030219) on Windows 10 (TH1 Professional 10240) 2) Disable Title Bar in Customize (default setting) 3) Install complete theme in AMO. (e.g. LavaFox V2 2.3.8, Noia Fox 3.1.4, Qute 5++ 1.5.0) 4) Enable the complate theme Actual results: Titlebar button doesn't work. It works with default theme, but not compatible with AMO complete theme. Expected results: Titlebar button works without developer effort.

All these themes themselves mess with the titlebar buttons and titlebar (setting the custom buttons to display: none, but still displaying the custom titlebar which then eats mouse events for the "real" titlebar buttons). The fact that it breaks is not Firefox's fault. We're going to eventually work on making light-weight themes more capable and/or finding some other happier medium between complete themes right now (which are very error-prone) and light-weight themes (which are a little underpowered right now). But here specifically, there isn't really much we can do.