Mozilla requires a unixy dev environment. Makefiles, hg, patches, ssh, irc, etc. Unfortunately Windows lacks in terminal abilities. No tabs, crappy cut/paste, limited size, slow, etc.
Emacs has a wonderful shell-mode(ie my bottom-left buffer). This means that windows can now do proper command history, cut/paste & use multiple emacs buffers(like tabs in a more sane terminal). Like all good features in emacs, out of the box shell mode is pretty busted, but can be fixed to default to bash. Microsoft also provides a very nice fixed width font for emacs. I keep my emacs config in hg. I really appreciate the operating system that is emacs.