Siphon is a super cool freeware addon for the Firefox web browser that lets you synchronize your addons across different computers. It ships with the option of a default online webserver to store your addon list for synch, or you can set up your own server and DIY. Unfortunately this part is still a little too geek for most people, as you need to configure Apache/MySQL et al in order to get it running.
It would be better to have the option of setting up a simple FTP type server alternative on your personal web space, with an easy to use wizard, but maybe that will come. The software works superbly - just install, set up an account, press synchronize and your addon info is all stored online ready for installation later on a different/new computer. Note that it doesn’t store your addon options settings or user data, so beware.
Nevertheless this is an essential tool if you use Firefox on multiple machines or reinstall it often. That is all.
Siphon will manage and sync all your Add-ons with your different computers. It will also save your Add-on list if you ever need to reinstall Firefox.