Oh, it seems Firefox/GTK needs fixing. It's trying to use both X11 (bug 1668429) and native Mir:
Feb 27 14:31:01 kg-Studio-1737 firefox[6135]: /build/gtk+3.0-MH7ynA/gtk+3.0-3.22.7/./gdk/x11/gdkwindow-x11.c:5573 drawable is not a native X11 window
Feb 27 14:31:01 kg-Studio-1737 firefox[6135]: Ignoring unknown Mir event 11
Feb 27 14:31:01 kg-Studio-1737 firefox[6135]: Ignoring unknown Mir event 8
Oh, it seems Firefox/GTK needs fixing. It's trying to use both X11 (bug 1668429) and native Mir:
Feb 27 14:31:01 kg-Studio-1737 firefox[6135]: /build/ gtk+3.0- MH7ynA/ gtk+3.0- 3.22.7/ ./gdk/x11/ gdkwindow- x11.c:5573 drawable is not a native X11 window
Feb 27 14:31:01 kg-Studio-1737 firefox[6135]: Ignoring unknown Mir event 11
Feb 27 14:31:01 kg-Studio-1737 firefox[6135]: Ignoring unknown Mir event 8