modules: firefox-profile: init.zsh clean up

Cleaned up init.zsh, so the whole initialization code is handled by
an anonymous function. This is make variable clean up much easier.
FIx the version string (bumped to 3.0, 2.x is standalone script
that can be sourced in ~/.zshrc (see my dotfiles repo for that).)

Minor fix for mktmp() for non GNU/Linux users.