Bug#850720: firefox: Use pkg-info.mk to figure out source name and version

[Rohan Garg]

Hey
> This change most likely breaks building ESR and Beta.
>

Oh, can you give me more info on how I can check my changes against that?

> There also needs to be a build dependency on dpkg-dev >= 1.16.1.
>

The package already depends on dpkg-dev >= 1.16.1 according to the build-deps

Cheers
Rohan Garg