Session Manager extension also good :)

I also like the Session Manager extension.
You can save a whole browsing session and give it a name. It doesn't actually store pages like ScrapBook, it just stores the URL's so it can open up the tabs again just the way you had them. Handy if for example you're investigating something and want to continue later.
It also auto-saves and will restore the firefox state (all windows and tabs) after a crash
https://addons.mozilla.org/firefox/2324/