In some cases, such as the ESR package on Debian, Firefox can be
installed to a different directory.
This commit alters the Makefile so it's possible to specify the Firefox
directory from the command-line.
Instead of trying to override the .desktop entry of Firefox by
installing the custom one and adjusting XDG_DATA_DIRS, just wrap the
firefox binary directly and adjust PATH. The former did not work with
Phosh, and on second thought, wrapping the binary is what's really
necessary. The launcher would have been just another useless layer.
