We've fixed this issue on the Firefox side ~2 months ago. Here is the bug: https://bugzilla.mozilla.org/show_bug.cgi?id=1897379 It's possible that you have an extension installed that blocks e.g. Twitter's CDN. We've seen issues like that with the Disconnect Firefox extension.
So if I understand correctly, this puts x.com under the same entity as twitter.com so third party cookies are allowed between x.com and other twitter sites?
The bug we fixed was in ETP, an older mechanism in Firefox, which blocks cookie access for known trackers, based on a list. Only for that mechanism we consider a hand full of domains by Twitter / X to be the same party. This is so we e.g. don't block their CDN if you're on x.com. We still block them on sites not belonging to Twitter / X. This is especially relevant for ETP strict which does not only block cookie and storage access, but also blocks 3rd party loads from known trackers altogether. If we block Twitter's CDN on x.com the site breaks.