Apple Mac mini and Studio Display, problem with DRM under Firefox

I have a Mac mini M4 with Apple Studio Display in front of me. Firefox apparently has problems with this, as it does not play certain channels on Blue TV. Apparently DRM prevents this. Firefox on a Macbook M1 also has no problem with the DRM. And the two Firefoxes are synchronised!

The “Play DRM content” box is ticked. What else can you set to make this configuration work again?

Is it really because an external monitor is being used here? Safari is not bothered by this - it plays these channels without grumbling.