A few months ago, I put together some mockups of a scheme for making add-ons available through the Add-ons Manager.
Thanks to a lot of hard work from Dave Townsend, Laura Thomson, and the webdev team, you can, as of the latest nightly build, now explore add-ons from within the Add-ons Manager.
When you open it, you'll find yourself presented with a few recommended add-ons. You may see fewer than this in the near term, given that the manager will only recommend add-ons that are compatible with your version of Firefox, but more and more are compatible every day!

You can also search through the full addons.mozilla.org (AMO) catalog and install directly from the Manager:

When you install, you'll encounter a simpler confirmation dialog than in the past:

and, finally, we make it clear that a restart is required and give you a way to do it:

On top of these changes, the manager will now enable a Theme when you install it, so, one restart later, you'll see it in place.
This is a work in progress, of course, and there are some things still to come:
Please try it out and tell us what you think, either in comments here or in bugzilla (Firefox::Extension Manager). We're eager to hear what you think!
Just a little warning, in case you're jittery -- I'm going to start using this blog to talk about Firefox user-interface and user-experience design issues. Why? Well, I started working for Mozilla about 6 months ago, and it's about time I started writing about the things that are keeping me busy.
If you're one of the few people who read this blog for, well, any other reason -- be not afraid. I'll still be using this blog for photos and blowing your minds, and all the Mozilla-related content will be tagged as such ("mozilla") for easy identification.
Shh shh. It'll be OK.