If you're a Firefox add-on aficionado, the free Add-On Collector is for you.
The meta-extension brings in a number of nice options for keeping track of add-ons and themes--either your own or someone else's. You can create collections for personal use to keep track of your must-have add-ons, or put together a list to share with friends or the world. You can also subscribe to collections created by Mozilla and other Firefox users.
It takes a little setup to first create your collections and begin using the Add-On Collector, starting with setting up an account for Mozilla's add-on site. Once set up, you'll have a new Subscriptions tab for the collections you're tracking, both your own and those shared by other Firefox users. A Web Developer's Toolbox, Traveler's Pack and Reference Desk are just some of your subscription options for existing collections.
To build your own collection, click a new "Publish to" button next to any add-ons or theme. Select an existing collection, or create a new one that can be for just you, your chosen friends, or the world.
You can also have Add-On Collector automatically maintain a collection of all the add-ons you have installed on a particular browser. Under the Auto-publisher Settings tab for the add-ons options, pick a name for the collection, whether it will be public, and whether to also include themes, dictionaries or language packs. Then click a "Create Auto-publisher" button, and you'll have a list to refer to any time you have to re-install Firefox, or install a fresh browser on a new PC.
When combined with other synchronization and cloud-storage tools like Xmarks for bookmarks and LastPass for passwords, the Add-On Collector frees Firefox from the confines of one PC. With these useful, free tools, you can seamlessly maintain the same custom setup for multiple browsers.