Thanks. Right, that's it, I'm probably on my way back.

It's actually had it for years (probably even always, almost certainly since about 1.5) via middle-clicking the refresh button.

Well, Firefox has existed since before tabs were a thing in browsers, so can't have been always, but yeah, wouldn't surprise me, if this was relatively quickly added after the introduction of tabs.

These kind of hidden features don't require a whole lot of designing and forethought, so it's something that usually only really needs one dev thinking that it'd be nifty to have them implement it.