It's not a feature and you can't disable it, previously it'd just crash. I think it's to do with the content workers being `exec`d from disk each time instead of forking from a master.
It used to crash under Linux, when you updated it while it was running, and then opened new window or a tab. Now it shows a message that you need to restart.
I restart my browser at most once per week and Firefox has never crashed for me after an update. I do not doubt that there was a risk that it could happen, but it cannot have been that common unless I was very lucky.
It wasn't immediately after update. It was after you updated it while it was running and then you tried something, that triggered the crash (like opening a new tab).
Later it stopped to crash, but acted weirdly (clicks on links didn't work, etc).
Nowadays we have the notice to restart the browser. It didn't arrive years ago, just a few releases ago.
This was only under Linux, and only with the distro package managers. Under Windows or MacOS, the updater updates the browser on next restart, so this didn't happen.
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