Note: The below directions apply to Firefox 2 or older. More updated directions canbe found at The Firefox Extension Guru’s Wiki.

Here is a neat little trick to open the Add-Ons and Download Managers into the sidebar instead of a new window:

1. From the Bookmarks Menu, select Organize Bookmarks
2. Click the New Bookmark Button
3. In Name field enter what you want to call this bookmark (Add-Ons or Downloads)
4. In the Location field copy/paste the appropriate address below:
   • Add-Ons: chrome://mozapps/content/extensions/extensions.xul
   • Downloads: chrome://mozapps/content/downloads/downloads.xul
5. Check the box: Load this bookmark in sidebar
6. Click OK

To make it even easier to access the Add-Ons & Download Managers, you can drag the bookmarks from the Bookmark Menus onto your Bookmark Toolbar.

Tip Source: Mozilla Links