(http://www.techspot.com/fileshost/newspics3/2010/firefox-newaddonsmanager.jpg)
Mozilla is working on an overhauled Firefox Add-ons Manager that should rollout with v3.7. Rather than opening as a tiny window separate from the rest of the browser, the revamped manager launches as a new tab. In addition to being in the Tools menu, you can also launch the manager by typing "about:addons" in the awesome bar (which inherently means you can create a handy bookmark shortcut).
The tab is home to two panes: one has a sidebar for navigating extensions, themes, plugins and other add-ons, and the other contains a browser, meaning that you can finally find, deploy, and manage add-ons from a single location.
Naturally, since the update isn't finished yet, the end product may have a different look and feel. Nonetheless, you can download the latest build here. It's worth noting that the new Add-ons Manager uses JavaScript, so if you're running NoScript you'll have to whitelist the page.
More info:
http://mozillalinks.org/wp/2010/04/meet-the-new-firefox-add-ons-manager/