These icons are based on Google Material Design.
The icons in this package are based on Material Design Iconic Font, which has added some additional community icons:
Thanks to Warren Buckley for creating the Belle Icon Pack: http://our.umbraco.org/projects/backoffice-extensions/belle-icon-pack and sharing in his blogpost how to extend Umbraco 7 with custom icons: http://creativewebspecialist.co.uk/2013/12/01/quick-tip-adding-custom-doctype-icons-to-umbraco-7/
There seems to be issues with Firefox to display the added font-icons (not the core icons). In Chrome and Internet Explorer it isn't a problem, so maybe Firefox has an issue with loading the css file and/or the font-files.