I think it's actually an understandable strategical move from Mozilla. They might loose some income from Google and probably have to cut the staff. But to keep the development of Firefox running they want to involve more people from the community and GitHub is the tool that brings most visibility on the market right now and is known by many developers. So the hurdle getting involved is much lower.

I think you can dislike the general move to a service like GitHub instead of GitLab (or something else). But I think we all benefit from the fact that Firefox's development continues and that we have a competing engine on the market.