Update Firefox logos

Further details:
* http://blog.seanmartell.com/2013/06/27/rebuilding-a-simplified-firefox-logo/
* http://www.mozilla.org/en-US/styleguide/identity/firefox/channels/
* https://blog.mozilla.org/creative/2013/06/27/a-new-firefox-logo-for-a-new-firefox-era/

Fix #25.