[dathinab]

Firefox only cares about Firefox working with the specific rust versions they intend to support. They also use some hidden non document rust flags to use unstable rust options/features on stable (but increasingly less, the flag exists only for bootstraping rust itself and isn't meant to be used by anything else).

Which is fine for Firefox itself.

Just a problem for anyone forking of Firefox. (Or for other reasons trying to build it with a different rust version.)