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Transmission and Firefox integration does work 100% - the Torrent Options dialog box does not popup automatically when you add a torrent via magnetic link or download a .torrent file and choose "Open with Transmission" in Firefox.
Maybe this is related to mutter and gtk3 - I've been experiencing this issue since the 3.20.x branch hit the official repos.
pacman -Q gtk3 firefox mutter transmission-gtk
gtk3 3.20.6-1
firefox 46.0.1-1
mutter 3.20.2-1
transmission-gtk 2.92-1
Fundamental Axiom of the Universe (aka Murphy's Law): Whatever can go wrong, will go wrong.
First Digital Deduction: Nothing obeys Murphy's Law so well as computers.
Second Digital Deduction: Everything go wrong at least once.
Third Digital Deduction: Things go wrong even when there's absolutely no possibility of anything go wrong.
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