In a word, no.

You're implying that Microsoft would find some reason to shut the project page down. First, I don't think they would.

Second firefox is entirely open source, so worrying about trade secrets is no concern. Next, code is in git, so the project could be spun up elsewhere, even if import and export of issues and the association of user accounts would be a one-time hurdle.

What’s the risk?

That Microsoft will disable the repo and prevent updates?

Or that they’ll read the open source code published to the repo?

Well. One risk now is that Firefox will become a part of copilot, and thus its code will feed Microsoft browser development regardless.

This is not what open source is supposed to mean.