At this point I don't see any open regressions from this behaviour, it's 5 years old, and it's an important security mechanism. I don't think there's a good reason for people to turn it off anymore. Let's get rid of the pref.

Double-checking with mkaply if there's any enterprise-y reasons that more control is needed (I would have thought that enterprise can probably add permission for their own sites using existing mechanisms but idk for sure if our policy work is generic enough for that at the moment.)

This pref was never made available via enterprise policy (we don't allow all security.* prefs)

So you can remove

https://hg.mozilla.org/mozilla-central/rev/262d41c4f299