The extension manager or add-on manager in Firefox has gone through some changes over the different versions of Firefox, however, it has not changed as much as people would have wanted it to. However, it looks like the next version of Firefox, which will be Firefox 4.0 instead of Firefox 3.7, will have a redesigned Extensions/add-ons Manager.
A new Wireframe for the extensions manager is being discussed at the Mozilla wiki. The new add-ons manager will have a sidebar which will provide easy access to installed add-ons, and also allow users to quickly search and browse other add-ons in the Mozilla repo.
The new UI for the add-ons manager is definitely refreshing, and having something like this in the next version of Firefox would definitely be great. If you want to take a look at how the Wireframes and other design related changes in the new extension manager which is being planned, head over to this page at the Mozilla wiki.
Do let me know what you think about the new wireframes for the extension manager? Do you think it will improve your experience while dealing with add-ons in Firefox?