When we grant an optional permission, we broadcast that to all existing child processes, but we never update the serialized values stored in Services.ppmm.sharedData.

Under e10s, it was easy for this to never have a real effect, we would quickly start the maximum number of content processes (between 1 and 8), and keep them alive till the browser is closed, never starting new ones. Also, the extension process where changes to permissions are most noticeable would have been already started, and never killed.

However with fission, most new tabs start a new process, it should be much more noticeable, and I'm really surprised we never got a bug report.

https://hg.mozilla.org/mozilla-central/rev/4c9a0604dee6