Enabling Wayland support by default prevents use of XWayland on Wayland
systems, while correctly falling back to X11 when Wayland is
unavailable in the current session.
With the current packaging many people unnecessarily rely on the
`firefox` attribute, which is suggested by nixos-generate-config, which
in turn makes their Firefox use XWayland, when it shouldn't, which
causes bugs with GNOME on Wayland:
https://discourse.nixos.org/t/firefox-all-black-when-first-launched-after-login/21143
Using the Wayland-enabled Firefox was tested on pure X11 systems by
contributors on the #nix-mozilla:nixos.org room and we are confident
this change will not cause severe regressions.
Even better, people can now toggle `MOZ_ENABLE_WAYLAND=<0|1>` in their
environment to override this decision, should they feel the need to do
so.
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