Previously, the Add-ons Manager operated in its own separate window apart from the rest of the browser. The new Add-ons Manager opens in a new tab and uses JavaScript and a sidebar to display extensions, themes, plugins and other add-on goodies.
Search engines have also been added to the new Add-ons Manager, which essentially means that all of your Firefox extras will finally be accessible and manageable from one place.
Right now, the Get Add-ons website hasn't been fully integrated with the Add-ons Manager, but it looks like the idea will be to fully integrate search and install capabilities on the fly.
The UI changes that you see right now might not be what appear in the final build, so keep that in mind if you want to test for issues today on the #testday channel at irc.mozilla.org.
If you want to get a look at what the new new Add-ons Manager looks like now, you can download the latest nightly build of Firefox 3.7.
[via Download Squad and Mozilla Links]