Rated 5 out of 5 stars

le Jack ar Aib. 8, 2015 · buan-nasc

I've been happily relying on this simple little workhorse since it was first brought to my attention that such a basic feature was missing from firefox. It does exactly what it says, with no excess baggage.
FF terminated it with prejudice, though, so https://addons.mozilla.org/en-US/firefox/addon/duplicate-in-tab-context-menu/?src=userprofile seems like a decent replacement.