I'm fairly sure Microsoft is actively trying to screw Firefox. Outlook has always sucked in Firefox, teams is a shit show. When you use a useragent switcher somehow a lot of features seem to work magically in Firefox (which tells me MS is doing this on purpose).
For Outlook (exchange) I use Thunderbird with a paid plug-in (to make the 2FA stuff work). It's pretty cheap and totally worth it for me at least.