Firefox already uses an external process to run Flash so leaving that on GTK2 while the browser uses GTK3 isn't an issue. The equivalent of XEmbed (to put the f

Firefox already uses an external process to run Flash so leaving that on GTK2 while the browser uses GTK3 isn't an issue. The equivalent of XEmbed (to put the flash content in the page) in Wayland is writing a nested Wayland compositor which would be compatible with an XWayland window. Basically, Firefox itself can be a native Wayland app while still using the X11 dependent Flash player.