That's fine. If they can't manage to make a service work using appropriate web standards then I don't think I trust them with hosting a service, my data, or to create adequate products anyway. -- I mean, what other corners are being cut?

It's quite probable that they are not cutting the corners that make them money and they're cutting (or might cut) everything else. After all, Firefox Nightly's share is a rounding error for Teams (and its users probably not Team's target anyway)

Yes. but things like backups or security will not make them money either. So I can’t assume that they're protecting my data.

Seems like a strange business decision to give up a a percentage of the market to the competition like that.

The percentage of the market that holds a strong opinion about a nondefault web browser with already minuscule (and consistently declining) market share, and will actively avoid you for not supporting it, almost definitely costs more to cater to then to ignore. (Testing time, engine specific bugs and code, etc.)

That small percentage will be extremely loyal to whichever services will cater to them though. A small percentage of users all choosing to use your service can still be quite a lot of users if there is no real competition in the segment.