It’s not a big deal since git repos aren’t hard to migrate. GitHub is fine currently and if they push people away then there are a couple of alternatives.
Firefox hosting on Github is a good move because it lowers the barrier of entry for contributors.
Moving to git is one thing, but doesn't going to GitHub put all their code at risk for CoPilot AI mining by Microsoft? (If one considers that a bad thing, which many don't, I guess.)
Your code is AI mined regardless where you put it today I'm afraid to tell.
Unless you put your code in a private repository self hosted behind a login. However, if your code is public. You can bet it will be used for AI training. Again regardless of which platform. And regardless which LLM. So all platforms, all internet, all LLMs.