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.