I have tried multiple times with my Hung Jury with Firefly on different enemies, single target, multikills and the solar Elemental Well doesn't pop up. Explosive Head, Explosive Payload, Timed Payload works, Dragonfly works, Stasis shattering works, even Black Talon's caster wave works, all proc the Solar Well. And yet, Firefly does not proc it. That's weird isn't it? It should naturally proc it since in-game description the Firefly explosion is always solar by design.