Firefox Sync has been around for quite a while now, however since becoming a part of the browser in Firefox 4, it has not seen many major updates in Firefox 5, 6, 7, 8 or 9. The good news is that this is about to change.
First of all, many changes to the Firefox Sync setup experience are coming in Firefox 10, where developers are striving to make it less intrusive, as simpler to understand.
Firefox Sync aims to synchronise the way you use and experience Firefox across multiple computers, and for this reason it synchronises even your browser history, since it forms an essential part of how the location bar (or AwesomeBar in Firefox lingo) works. However Add-ons, perhaps one of the most essential aspects of the Firefox experience, were not synced till now. In Firefox 11, this is about to change.
Firefox 11 will add Add-ons among the type of browser data that can synchronised. There have been ways to synchronize Firefox add-ons before, for example the Siphon Firefox add-on, or even using Mozilla's own Add-on Collector, however both rely on add-ons to add that functionality, so it isn't exactly out-of-the-box, and you need to create yet another account for syncing add-ons.
The way add-on sync is implemented in Firefox right now, prevents most abusive uses. It will not sync all extensions, but only trusted ones hosted on Mozilla's add-on repository. This is to ensure that a malicious add-on added to one Firefox sync instance will not spread to other Firefox instances. Also it saves from actually storing the add-on on Mozilla's sync servers, as they can be pulled from the add-on repository instead. Another important thing to note is that custom files and preferences of add-ons are not synchronized.
Firefox 11 will release in March 2012; till then if you are keen to check out add-on syncing in Firefox, download an Aurora build of Firefox 11. You can learn more about the feature from its page on the Mozilla wiki.
Unless you are a new Firefox Sync user, you will need to manually enable Firefox Sync for add-ons from the Sync preferences.