Here is what the folks at Diggo have to say about the extension.
Several powerful features are combined in the Diigo toolbar to make this a real “power extension” and must-have for anyone who browse a lot of stuffs online. An All-in-One Bookmarking Tool: bookmark to Diigo, delicious, Simpy, furl, spurl and make them permanently cached and full-text searchable. A Powerful Blogging Platform : Annotate webpages and quickly turn them into blogs with a built-in blog editor or enhanced linkrolls. A Great Collaborative Tool: share and interact on online findings, complete with highlights and sticky notes. The Most Customizable Search Tool: like Google’s toolbar, but fully customizable, so you can add any other specialty searches - dictionaries, music, movies, references, and maps.
Here is a comment I found via the Mozilla Add-ons Web site that I think fits Diggo perfectly.
Diigo is the best bookmarking tool that I’ve tried. And this extension is a must! You can bookmark a page right from your context menu, which includes highlighting parts of the page when you bookmark, and you can also forward a bookmark to someone.
I don’t even keep my bookmarks in my browser anymore - I got tired of looking at 100+ jumbled bookmarks in my browser, so now I use the Diigo extension to retrieve them. I like being able to bring them up as-needed, but the rest of the time they’re stored at my Diigo homepage.