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.
That’s exactly what it is. Firefox’s advanced tracking protection blocks connections to social media sites from other sites so that social media can’t see your behavior on the rest of the Internet.
Twitter started moving some things to a different domain and FF saw it as a third-party, blocking connections from it to the old Twitter domains.
Yet another reason the rebrand is dumb.