Microsoft Outlook 365 has its own default browser setting.

I use Thunderbird, not Outlook. Damn, Edge hijacked your answer - opened in Edge

Does it still happen in Troubleshoot Mode?

It's not that Firefox won't open a link if pasted into Firefox; the problem is that every link in email now opens with Edge. Tried deleting Edge; that didn't work. Even your message opens in Edge - never had this problem until yesterday.

Does Firefox identify itself as the default browser?