This is how Mozilla's own updater works (if you use the .tar.gz version), but the distro package updater just overwrites everything without waiting and applications like Firefox have no control over that. As a user, you'd really want to disable unattended updates for such software.
It's not an issue for these .deb packages because they enable the (experimental) forkserver.
Though it's possible they use different code injection tricks to make blocking impossible. (You can't block Defender from listening to events for example)
One reason why it shipped on Windows first is that the situation is more serious there. The article goes into details why the different overcommit behavior is more problematic on Windows, and you're more likely to OOM there as a result.
Getting macOS to OOM is actually pretty difficult, we probably want to unload tabs before that point for performance (swap!) reasons, but that's also harder to measure very reliably.
Because for some people their open sites, and the ability to switch quickly between them, is the most important use of their machine and what they want their RAM to be used for.
It wasn't enabled by default because it didn't work well enough. Getting better low memory triggers was a key to making this an overall benefit, and not just a user annoyance.
Not sure if it's a coding issue in the Firefox version of Dark Reader, or it's hitting some slow path in Firefox itself.