I'll try to document how to set up OS X to compile Firefox -- in order to have all the relevant links in one place.
Please note that his information might became obsolete, please check the date of the post
Setup the environment with the bootstrap script (link in MDN)
Python will be installed from Homebrew (since Python >= 2.7.3 is required). If link fails because pip executables will be overwritten, uninstall pip using pip uninstall pip (sudo may be needed)
build with mach
the tool is refined, presenting clear feedback about progress