Currently, Fenfire is best applicable to browsing and editing RDF files. You can use the same features to view some Semantic Web data and RSS feeds of weblogs, for instance. Keeping notes or such is also possible, but requires more technical understanding than the goal would be.
Reading PDF files is almost usable as well, and keeping an archive of linked articles and notes about them is not far away either.
If you run Gnowsis, Fenfire can integrate to its services. This means that Fenfire is able to both ask for data about items from Gnowsis, and tell Gnowsis to open items such as email messages and web pages.