Siphon – Add-on Synchronizer for Firefox
You have Firefox on several machines and you want to have the same add-ons everywhere. Also, when you re-install a machine, you want to have back your favorite add-ons.
“Siphon will manage and sync all your Firefox Add-ons with your different computers. It will also save your Add-on list if you ever need to reinstall Firefox.“
Read about Siphon on Lifehacker.