Google Browser Sync!
Firefox’s major asset is its wide variety of user community created and supported extensions.
The ability the save all open windows and active tabs was a “cannot live without” feature brought forth, in part by Tab Mix Plus. Firefox 2 now incorporates similar functionality. The ability to sync bookmarks across various installs using the Foxmarks extension greatly eased the transition between desktop, laptop, and/or virtual machines as well.
Google, being Google, has upped the ante with “Google Browser Sync for Firefox ":https://addons.mozilla.org/addon.php?id=1122. (Verbatim) Google Browser Sync for Firefox is an extension that continuously synchronizes your browser settings—including bookmarks, history, persistent cookies, and saved passwords—across your computers. It also allows you to restore open tabs and windows across different machines and browser sessions.
A valuable extension to save session feature between computers, such as those users who use a desktop and laptop, or Windows machine and Mac OS machine.
Session Manager allowed for more powerful session saving than Tab Mix Plus, not only loading whatever sites you were visiting but the position of the scrollbar, contents typed into textfields, etc. At the moment this extension is not available for Firefox 2.0