Today we are announcing the end of official support for the Firefox Sync add-on, focusing resources on improving and supporting the Sync experience for those users in the latest versions of Firefox. This means that the add-on will not be available at addons.mozilla.org and the only way to get Firefox Sync features will be to upgrade to the latest version of Firefox where they are built into the browser.
A lot has happened since Mozilla launched Weave back in 2007: We launched Weave as an add-on, rebranded it as Firefox Sync, moved Firefox to Rapid Release and eventually integrated Firefox Sync into the browser. Our goal has always been to offer the best service so you can seamlessly share your Firefox experience across devices
Since the launch of Firefox 4, the browser comes bundled with all the goodness of Firefox Sync. Because of the continued support for Firefox 3.6, we have continued to support the add-on. This causes a great deal of engineering & testing headaches between the add-on and the integrated Sync. We also see a lot of users unhappiness since the add-on does not support some of the newer, highly desired, features we’ve added to Sync.
Keep in mind the add-on will still be usable for now, but we will not guarantee its reliability in the following months and foreseeable future. We ask everyone still using the Sync add-on to move to newer versions of Firefox to enjoy the best online and Sync experience.
— mconnor & ibai, on behalf of the Services and SUMO teams