Twitter/x.com is now forcing you to disable Firefox's Enhance Tracking Protection.
Firefox’s Enhanced Tracking Protection (Strict Mode) is known to cause issues on x.com
There were no "issues"; everything was working completely fine. This is a deliberate decision to force people to turn off tracking protection.
I saw a recommendation to use Firefox's container extension https://support.mozilla.org/en-US/kb/containers, but it's disabled in private browsing windows, and I always use private browsing windows.
saying they're forcing you to disable tracking protection is giving them too much credit.
they basically just botched the switchover from one domain to another and forgot about extra tracking domains etc so firefox flagged it as coming from a different domain.