I downloaded the Session one and it's pretty cool. Does basically what I need it to. Yitzter, you didn't mention which backup add-on so I'm not sure what you mean. I think there's one called FEBE but I don't know if it works for FF3.
I think with the Google Toolbar for FF, you can log into your google account through the toolbar and from there you can have all of your bookmarks saved. Not only that, but it'll save all your browsing history, no matter which computer it was on, as long as you're logged in. Good for people who have a computer at work and one at home.