Since YouTube has massive resources and famed tooling&processes, presumably either they knew about the Firefox problems before deploying, or (if there was a genuine QA mistake) they'd know about it very soon after deploy, when they'd be able to rollback if they wanted to.
(Obviously, they know about Firefox, they've developed for it since it was available, and they've even been funding it.)
(Obviously, they know about Firefox, they've developed for it since it was available, and they've even been funding it.)