After some more playing around it seems it's working fine, just that the Popup Blocker is doing it's job too well! Went back to version 4.4.5.1000 to see when it stopped working, only to notice the popup blocker counter increase. It's always the weirdest things! Not sure why it works on MX sites, unless the popup blocker is disabled for these sites even though it still shows as enabled?
Was hoping to extension-ify it, (it actually works well + a context menu option). But unless there's a way to make it bypass the popup blocker without having to disable it, it's pretty much useless.