Windows/Mac/Linux (Firefox): If you've got multiple Firefox installations across different computers or just find yourself setting up Firefox often, Siphon is a must-add. This extension simply syncs a list of add-ons between browsers, with easy installation and deletion buttons for each.
It doesn't seem as though Siphon syncs your add-on preferences or particular user data stores — just the list of extensions you've got installed. You can use Siphon's server by default or set up your own server through an API. Once you've created an account and logged in on any server, Siphon keeps track of what you've installed and later removed or added on — as you do, you can choose whether to sync those changes or keep them on the local Firefox installation only. That's what Siphon does, and while simple, it fills out a crucial missing piece of the Firefox Sync offerings.
Siphon is a free download that works with Firefox. The author states that it was tested primarily on Mac and Linux systems, and we tested it with a Mac. Windows users, if you find troubles, let us know in the comments.
Siphon [Firefox Add-Ons]