Xmarks Sync (formerly known as Foxmarks) is possibly my favorite Firefox add-on and has been a solid staple add-on for my browser ever since I started using it.
Xmarks Sync is a free add-on for Firefox that syncs and backs up your bookmarks across multiple computers and more. The result is that wherever you have Firefox, you have your bookmarks with you at any time. This is superb if you have a highly customized bookmarks bar like myself and you simply can't get by without it. What's more, you never need to worry about losing your bookmarks again because Xmarks always holds a backup of them on its servers.
One minor drawback is that you have to sign-up for an Xmarks account but it takes less than a minute during or before installation. Create a username, enter a password and within less than 30 seconds, all of your bookmarks will be synced by Xmarks. You'll see a small Xmarks Sync icon in your menu bar which allows you to synchronize Xmarks at any time and see your bookmarks on the Xmarks server.
The only downside of Xmarks Sync is that Firefox is prone to slowing down when bookmarks are syncing. It's effect on the stability of Firefox is also dubious although it's hard to say whether it's responsible for any crashes.
Xmarks Sync for Firefoxi is an absolute essential for anyone who regularly relies on their bookmarks across different machines. One of the best free add-ons you'll ever use.