User Agent: Mozilla/5.0 (X11; Linux x86_64; rv:43.0) Gecko/20100101 Firefox/43.0 Build ID: 20151221065651 Steps to reproduce: In ratpoison (possibly any tiling WM), double click on the tab bar where there is no tab. Actual results: Firefox freezes for a moment and crashes. Gdk:ERROR:gdkwindow-x11.c:5129:create_moveresize_window: assertion failed: (mv_resize->moveresize_emulation_window == NULL) Redirecting call to abort() to mozalloc_abort Expected results: In previous versions of Firefox, double clicking in that location opened a new tab. This behavior appears gone (confirmed under a separate window manager). So either: open a new tab, or nothing should happen.