[quietbritishjim]

I don't think that is a difference: for the feature my comment was about (popups in tabs), the chances of Firefox and Chrome rolling this out to all users by default (not in a config, arcane or otherwise) seems roughly equal to me. That is: neither currently shows any indication, and I can't imagine why!

I case I was unclear, the setting I'm talking about is totally different from the bug report linked by OP. For that: I have never seen a popup appear when closing a tab in Firefox so I suspect it was already fixed long before Chrome (and I haven't changed anything related to that in about: config)! But I haven't been about to find any definitive source about it.