PS:
This isn't worth the effort, however...
1. disable updating the firefox package:
/etc/pacman.conf IgnorePkg = firefox
2. write a wrapper for pacman that first compares the installed version, with the version provided by the repositories
pacman -Q firefox | cut -d\ -f2 pacman -Si firefox | grep Version | cut -d: -f2
3. use the sed command, so that the wrapper could edit the version mentioned by your PKGBUILD
A better idea would be to get the official PKGBUILD by coping it from ABS and then to add ac_add_options --enable-alsa maybe behind ac_add_options --prefix=/usr since the prefix unlikely will change.
4. let the wrapper build and install your package
5. let the wrapper run a regular "pacman" with $@ options
IMO it's easier to use apulse-git and if you should use freedesktop.org menus, to edit a desktop file to launch "apulse firefox" by a sript.