If I wanted notifications for new releases, I would permit notifications. If you cannot respect a user’s choice to upgrade at their own timing without being constantly harangued by a modal which obstructs the rendered page (and without implementing ridiculously arcane policy mechanisms for their personal installation), then you shouldn’t be in the software business in 2022!!! For an otherwise great product like Firefox, this is just plain bad design.
If you want users to upgrade, notify them ONCE for each release, at the next app launch, and if there’s a critical security issue, include the spec in the notification. You could even put one of those little red dots everyone uses to tweak their users in the window header or menubar.
It is pure abuse of the user to put a modal dialog in their face every time somebody does a minor fix. One has to wonder if anyone allowing UX like that into release even cares that Firefox is hemorrhaging userbase. Users do not want their software to get in the way of their productivity, and enabling automatic installs of new software versions as an alternative to the doorhanger is a sure way to break something in a way that leaves a user really pissed off because they have no idea what broke or why.