Oh yes the one I mean is Bypass Paywalls Clean (D). But this also seems to have been kicked out of the Mozilla store. It was just a resubmit by someone else after the original got kicked out when a French newspaper complained.
We really need sideloading capability in mobile firefox :(
Settings> about Firefox> tap on the fox logo a few times> debug menu enabled> now you see "install ad on from file" under advanced in the settings menu.
EDIT: Actually, never mind, this isn't even that, this is a fake according to the reviews.