You can find a list of 10 Firefox extensions (adds_on) including a reference desk.
Reference Desk is good for students or anyone who needs to research information.
SimilarWeb, DeeperWeb, Scrapbook, ReadItLater. Meriam-Webster etc.
Read Bob Rankin website and check his list of free reference and Web research tools.
You will find both web resources (encyclopedia, dictionaries etc) and tools. The list lacks of typical Web research or Web clippings tools (Evernote, Webnotes, Zotero, Ubernote etc) but you will find Web resources in different sectors, e.g. medical advic e, business and financial research, language translation, movies, music, genealogy, US government etc.