Those are the four most important information management addons I use in Firefox.

Stumbleupon is for channel surfing and finding cool new stuff. It’s also great for sending links and comments around to a group of friends. Unless they don’t ever click stumble: “The person you are sending a page to has too many sites waiting in their stumble queue. ” 

Delicious is for bookmarking, social or otherwise.

Read it Later is for storing one time sites (i.e. ones you might not want to necessarily bookmark) for later consumption.

and Zotero is the ultimate reference management tool and webpage snapshot addon.