TV+ in Safari or Firefox

I just signed up for the 7-day free trial of Apple TV+

I'm running a 2018 Mac Mini, connected to a projector, so I'd like to watch from a web browser, which Apple says should be possible.

I have a 50mbps fibre connection so speed shouldn't be an issue. I can watch Netflix without any problems.

Safari allows me to sign in and browse to an episode, but when I try to play, it just doesn't work at all. I get a Play symbol (triangle) with a slash through, which seems to indicate that the play function is not available.

If I log out and open Firefox and sign in again, the episode does in fact play. However, after about 10 minutes, it goes to a black screen and stops. I have to refresh the browser, restart the episode and try to find where I was watching from. (There appears to be no Resume function).

Anyone else having problems?

Any suggestions on how to achieve better results?

I'm running MacOS 10.14.6 and no, I don't want to upgrade to Catalina.