Showcase provides a new way to manage your Firefox tabs and windows by showing them as thumbnails in a single window, tab or sidebar. Includes a find bar that will filter the thumbnails, and the capability to select the thumbnails in the same way you would select files in your system.
To learn how to use Showcase, take a look at the manual.
In the gallery section, you can find several screenshots of Firefox Showcase in action.
From the download section any version of Showcase may be downloaded. Also you may find a version of this extension for Mozilla SeaMonkey and Portable Firefox, and the latest version of the MultiSidebar extension.
If you're facing any problems or have any suggestions, feel free to contact me.