To be fair, on IE it doesn't clone the entire window, just the current tab into a new window. Still, that's my main use case, and AFAICT it's not possible in Firefox by keyboard or even with a single mouse action.

There's a mouse shortcut for Duplicate Tab in Firefox: middle-click the Reload button! It blew my mind when I read about it in some random thread a couple of years ago.

I've been using it since more or less the start and didn't know that. Thanks!

Same for Chrome. This is amazing.

There's multiple extensions that claim to provide a shortcut to the duplicate tab option, fwiw

Thanks. Now that I looked into it again, the issue I have is not just the Duplicate Tab feature, but that opening a link in a new tab or window clears the history of that new tab/window instead of cloning the history of the source tab. that means that you can't quickly open a couple of links into new tabs/windows, with each of them preserving the current history. I don't think there's a fix for that.

Ah yes, I remember loving that feature in Netscape at the tail end, when it was just an altered clone of Firefox.

I got used to losing it eventually after switching over, but I do remember it being a painful transition for me too.