Perhaps Microsoft offered to pick up the tab that Google has been paying, but is now imperiled, or at least lend some sort of financial support, and Firefox cares more about paying their bills than open source
Even before this Mozilla almost certainly used hundreds of closed source tools, including things like Slack, Excel, Anaplan, Workday, etc.
Think you might be on something, with the incoming end of Google cash flow, Firefox may be in discussion with bing and that could be part of the agreement, use Microsoft server.
issues are stored in git bug and automatically synced. Github is the only viable option, but you can keep the others as mirrors when github chooses to strike you.
They should restructure instead, hire people who actually want to work on software and not use corporation and foundation around it as platform for their... peculiar "endeavours". But I doubt that's gonna happen - flow of Google cash and from all those naive people who think supporting Mozilla directly contributes to Firefox is too good it seems. But then it's understandable they do this - money from Google tap can get twisted.
Bad PRs all around, with just a constant stream of drive by "why no merge?!?!?!" comments.