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The solution there (adding an exception for x.com) works. I already had Twitter containerized.

Interesting... Twitter started working again for me as soon as I created a new container for it. I didn't have to add an exception.

I have Firefox's "Enhanced Tracking Protection" set to "Custom", with all boxes checked. And I'm running uBlock Origin.

Is it ? If you add an exception, won't you be tracked by all X icons on every other websites ?

Good point. You're isolating your logged in session but not the fingerprinting.