I would not even try to think how much work would that mean. This is the Android source they have: https://github.com/mozilla-mobile/focus-android - It seems it could be easier to start from scratch, creating a custom UI that works with Mir/Wayland and is just using the FFox backend. And it doesn't have to be called Firefox - https://en.wikipedia.org/wiki/Mozilla_software_rebranded_by_Debian#Iceweasel - Sadly that's quite a lot of work and commitment since it would have to be maintained and updated regularly.
But it would "work" to just click-package the regular desktop Firefox with Wayland support enabled. With little tweaks here and there it could actually be ok, especially on tablets. For example there were plugins for touch interaction a few years back which could help. The UI could be scaled up and when running natively with the latest Mir the performance should also be better.