It's bad UX but how far should Firefox go out of its way to support the situation where a (from its perspective) third-party package manager just deleted all its files, replacing them with subtly different/newer ones?
Unlike Microsoft and Apple, on Linux you're never forced to upgrade. Thus, when you do upgrade, you should really be checking what packages are upgraded. The alternative to such a page would be to require you to stop Firefox before the updates start. So it is still a trade off.