firefox: support user content CSS
firefox: support user content CSS

The `userContent.css` file is similar to `userChrome.css`, it's a CSS
file that you can use to change the way web sites and e-mails look.

See http://kb.mozillazine.org/index.php?title=UserContent.css

PR #1079

rycee/home-manager,rycee/home-manager,rycee/home-manager