I use AwesomeWM as my window manager of choice. As you probably know that’s tilling window manager. If you also use it you know what PITA it is to have fullscreen flash in Firefox, especially if you’re using multiple displays. So here are the things that I did to hack around that problem:

  1. Add following code in your rc.lua:
{ rule = { instance = "plugin-container" },
properties = { floating = true } },
{ rule = { instance = "Plugin-container" },
properties = { floating = true } },
  1. Apply dual monitor hack: sed -i -re s/_NET_ACTIVE_WINDOW/XNET_ACTIVE_WINDOW/ /usr/lib/mozilla/plugins/libflashplayer.so

*Location of libflashplayer.so file may vary, to locate it on your system you may use command locate libflashplayer.so