Do they depend on the equipped ship's firing arc? Example: Can a firespray fire heavy lasers behind it?

Secondary weapons can only fire from the Primary Arc, unless stated otherwise. So no the Firespray can't shoot the heavy laser on it's rear arc.

Also just because it often comes up. Ships like the YT's or VT's have a primary arc like all other ships, they just can fire their primary outside that arc. So if you give the YT-1300 a missile it can only fire it in the primary arc.

Yes they depend on the ship's primary firing arc.

The Firespray has an auxillary firing arc: one that cannot shoot secondary weapons, but in every other way counts as a firing arc.

Falcons and whatnot simply ignore their firing arc for that particular weapon, but still rely on the firing arc for secondary weapons, tactician, et c.

Alrighty. Thanks for the info.