User Agent: Mozilla/5.0 (X11; Linux x86_64; rv:69.0) Gecko/20100101 Firefox/69.0

Steps to reproduce:

1. Launch Firefox on Linux using the command env MOZ_ENABLE_WAYLAND=1 firefox
2. Launch Firefox on Linux using the command env MOZ_ENABLE_WAYLAND=0 firefox

Actual results:

1. Firefox runs on Wayland
2. Firefox runs on Wayland

Expected results:

1. Firefox runs on Wayland
2. Firefox runs on X11 (Xwayland)

I tested this behaviour on Gnome Shell.

Source code seems to match the description.

    // Make X11 backend the default one unless MOZ_ENABLE_WAYLAND or
    // GDK_BACKEND are specified.
    disableWayland = (PR_GetEnv("GDK_BACKEND") == nullptr) &&
                     (PR_GetEnv("MOZ_ENABLE_WAYLAND") == nullptr);

Yes, please use "unset MOZ_ENABLE_WAYLAND" to disable it.

(In reply to Martin Stránský [:stransky] from comment #2)

Yes, please use "unset MOZ_ENABLE_WAYLAND" to disable it.

I use "MOZ_ENABLE_WAYLAND" to launch Firefox on Wayland. However, when I want Firefox to use X11, I don't use this variable. It just seems illogical that both values do the same job. It might be a good idea to implement a new variable like MOZ_BACKEND for setting a needed backend.