And also with the atomic/immutable distros, the switch is practically instant, so it's not even like it forces you to watch a spinning circle for 20 minutes when you turn off your computer. You reboot and the apps all start clean with the right library versions.
It's rare but I've seen software trash itself because the newly spawned process talks a different protocol and it can lead to either crashes or off behavior that leads to a crash eventually. Or it tries to read a file mid update. Kernel updates can make it so when you plug in a USB stick, nothing happens because the driver's gone. Firefox as you mentionned. Chromium will tolerate it mostly but it can get very weird over time.
The risk is non-zero, so when you target end users that don't want to have to troubleshoot, it's safer to just do offline updates. Especially with Flatpaks now, you get those updated online and really it's only system components you don't care to delay updates taking effect
If you're new to Linux and everyone told you you can just update and no reboot, and you run into weird Firefox glitches, it just looks bad.