The new Firefox 4 features the Weave Sync Addon which allows you with something which was earlier not offered by any other browsers. The Online bookmarking service with sync on multiple computers or mobile device based browsers. There was a time when computer users would use Online bookmarking services for storing their bookmarks and use the Password management services to save the passwords and then use them through different devices but with the Weave Sync add on you can perform all this stuff from a single location.
This free add-on for Firefox 4 allows you to stay in sync and get access to your website access history, stored passwords, saved bookmarks and also the currently active and opened tabs on all your devices.
The addon would automatically sync and keep all your stuff ready when you access the browser from your mobile phone at the same state where you left it on your desktop giving you a backup of bookmarks online. This means that you dont loose any of your currently active tasks and get continuous synchronization.
Features List :
- Get the same results on the Smart Location Bar on each of your Firefox browsers, so you can get to your favorite sites with just a few keystrokes
- Continue what you were doing: have the ability to open any tab you have open on any of your Firefox browsers
- Keep the same list of bookmarks on all of your Firefox browsers
- Easily sign in to all your favorite sites using your saved passwords (this is especially handy on mobile phones, where it’s hard to type in complex passwords)
- Do it all securely: Weave Sync encrypts user data before uploading it to Mozilla’s servers, so that only you can access your data

Is it an add-on? On my Firefox 4 beta 5 it was pre-installed, and doesn’t show in the add-on management page… so hasn’t it become a plain standard feature of the browser?
Yes, it will become or already is a standard feature of the browser