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