Heads up I was stuck on 3.3.x even though I had auto-updates on. Had to manually update to 3.4.2 which installed as a separate extension rather than replacing the old one.
Overlay still opens in a new tab, but at least I am current.
As others have said the Chrome extension is much more polished/feature rich, and it would be great to see the FF extension get the attention it deserves, particularly with `Quantum` released and now v60.