Earlier quoted context omitted.
In my experience, most contributors who are deterred from contributing because they can't use GitHub aren't particularly valuable contributors. I'm sure there's exceptions, but I haven't seen any for non-trivial open source projects I've been involved in. I might even argue that it could be good to have a slightly higher bar to deter low quality one time contributors.
Contribution isn’t driven by a desire for rewards, but by goodwill. Friction only gets in the way. If the friction is worth it, fine - but what exactly is being lost by moving the repository to GitHub?
Alternatives to github
We lament Google's browser engine monopoly, but putting the vast majority of open source projects on github is just the expected course to take. I guess we'll repeat history once microsoft decides to set in the enshittification, maybe one day mobile OSes replace Windows and they're strapped for cash, who knows, but it's a centralised closed system owned by a corporation that absolutely adores FOSS
I don't mind any particular project (such as this one) being in Github and I can understand that Mozilla chooses the easy path, they've got bigger problems after all, but it's not like there are no concerns with everyone and everything moving to github