They've been rolling out rapidly varying and inconsistently-behaving (between users/refreshes) changes in a pretty explicit attempt to break adblockers and complicate developing new ones. You're probably either just not in that cohort yet for some reason, or have already received the improvements that keep ubo working - people's experience varies widely with this wave, probably by design (difficult for blockers to study) and to try to judge impact.

They really only did it on chrome, I think?

At least at work, I kept getting modals saying adblockers weren't allowed, but I never got those on firefox.

It seems to have gone away though (probably an update from ublock) even on chrome

No, their adblocker detection definitely detects UO in Firefox. I've had to disable it (UO) the last couple months otherwise YT becomes unusable.

Huh, fair enough. Guess it just missed me

I've seen those on Firefox. Disappeared after a week-and-a-half though.