However, this has left Firefox unable to access the site by default.

You will instead get a message about "Something went wrong, but don’t fret — let’s give it another shot."

This seems to be due to the manual allowlists FF has as part of the "Enhanced Tracking Protection" feature. It doesn't know that X.com should have access to the Twitter image distribution URLs and other non X.com URLs, so the page load fails. X access can be restored by turning off Enhanced Tracking Protection.

The Twitter engineering lead is asking for anyone who has a direct line to FF staff: https://x.com/cambridgemike/status/1790615764736913557