With Firefox "Enhanced Tracking Protection" turned on, the requests to fbcdn.net domains are blocked and I just get a blank page. With it turned off, it's just an abstract splash image with a 2-day countdown and a QR code that opens links to the mobile app stores.

Presumably at some point the trackers list Firefox uses will be updated to mark Threads as a Facebook property and allow those [fbcdn.net] resources to be loaded [there].

[edits]

I feel like "allowing Facebook properties" and "Firefox tracker protection" are fundamentally at odds.

It depends on what the goal is: in this case, fbcdn is first-party for the threads website (because they’re both owned by the same company) and blocking it makes for a worse experience.