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?
Firefox partitions (and soon blocks) 3rd party cookies by default. That means they can't be used for cross-site tracking. Also not across different top level sites belonging to Twitter / X. The entity configuration for ETP does not change anything here.
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.
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.