You can avoid this with a package system that doesn't interfere with running applications during updates like the snap or flatpak package.

I am using a Linux OS and do not have the issue you report. The package manager runs when I launch it and updates or installs what I tell it to update or install. I never update running applications.

All other libs, programs and applications are automatically updated in Mint without causing any disruption to current execution. This includes several browsers. Why is Firefox the only package that behaves differently? How could developers of other browsers avoid this issue if that is technically impossible like some suggest?

If you don't want to install the snap or flatpack package, you can install the official build and change the update setting to "Check for updates but let you choose to install them". It won't force a restart unless you leave pending updates for too long.

You could also try the new deb package but it is currently limited to Nightly builds.

Firefox can continue browsing after an APT upgrade, allowing people to restart at their convenience.

But why should I make an exception for this specific piece of software among the thousands of software packages that make up my OS?... Why do you try so hard to convince others this is not a wrong behavior from FF? What are you trying to achieve with this?

Multi-process browsers are unlike almost any other kind of software. Mozilla's DEB package proves that it can be updated by APT without forcing a restart if it is configured properly. It looks like this was addressed by changing dom.ipc.forkserver.enable to true in about:config (Bug 1836348). This setting was introduced in version 75 (Bug 1609881).