Comment # 1 on bug 907541 from Wolfgang Rosenauer

This cannot be done within Firefox.
Firefox uses gio to map file extensions to mime-types and after all to the
application. gio itself is implementing the freedesktop "standard":
http://freedesktop.org/wiki/Specifications/shared-mime-info-spec/

This works fine for me on 13.2 with WindowMaker and Gnome. DOCs are offered to
be opened by LibreOffice.
Unfortunately I'm not that deep into gio and the underlying share-mime-info
handling.
Let's try the Gnome guys for feedback.