As a web developer, I have a lot of extensions in Firefox. I also get new computers (and create new test accounts) with alarming frequency and often install and set up Firefox for friends and family. I want to be able to create and share lists of extensions, like a music playlist, to make it easy to set up Firefox when I get a new computer, or give a pre-made list to my friends and family. Here are my thoughts of a product I'd like to exist.
1) You create a site -- say, my.firefox.com -- and I log into it. I can upload a list of all the extensions that I have. (Auto-generated by the browser, of course.) When I go to a new machine, I log in (once logged in, I'm looking at my.firefox.com/mynamehere) and I can click one button to install the current version of all listed extensions on my current machine. (Extensions that are already installed and current are not touched; present but outdated extensions are silently upgraded.) Or, if I only want a few extensions (like if I want to install my dev tools but not Adblock Plus or NoScript so I can see how the Web looks to the rest of the world) I can check a box next to each one I want and then click "install selected extensions" at the bottom of the page. Maybe, if you want to be fancy, there is a list of the extensions and a red/green indicator that shows if it's present on my current machine.
2) You know how music services let you create and share playlists? That's what I want to do with Firefox extensions. I want a list for my mom with nothing but Adblock Plus and Status-4-Evar so I go to the page that shows all my extensions, check the boxes next to those two, click "Create new list", and name it "mom". Then, from her computer, I go to my.firefox.com/mynamehere/mom (or call her up and tell her the address, or email her the link -- in any case, this is why I want clean, clear, and memorable URLs) and click "Install all". Maybe there's a form on the page that I can use to email her a link, like how you can email articles from news sites -- your basic "share" button. (Automatically-generated and -emailed links could be long and obscure.) Maybe my sister is a bit more tech-savvy -- but not a web developer -- so I make a list for her with ABP, NoScript, and a couple others, but not all of my dev tools.
And that's it. Keep it simple. No "click here to share this list on Facebook/Twitter/G+/App.net" buttons. No scrolling list of new or popular extensions. Just a nice, simple, extremely useful service. The front page would be nothing but "sign up" and "sign in". Maybe two more buttons for "Why is Firefox great?" and "What are extensions"? Once you're logged in, there would have to be some sniffing done to see if you're using Firefox or not, and enable/disable features based on that -- for example, there would be no "install" button if you're not using Firefox, but you could still manage lists.
Lists would not be visible unless 1) you're signed in as you, 2) someone got there via cryptic, generated URL (from the "share" box), or 3) you have checked a box that says "make this list public." That way people couldn't poke around and find out that a famous columnist for the WSJ or NYT has a list for 'family' with Adblock Plus.