Functionality Changed, then Obsoleted Rated 4 out of 5 stars
As gshumway said, the behavior of this add-on changed between Firefox 3.0 and 3.5 with no explanation. However, as many other users have commented on or requested, this add-on's functionality has been built-in Firefox 3.6! Currently (3.6b4), the functionality is disabled by default, so you have to enable it in about:config. Change browser.ctrlTab.previews to TRUE to enable the Control+Tab functionality, and change browser.allTabs.previews to TRUE to change the "List All Tabs" button to show an "All Tabs panel".
Great add-on! It was ahead of the core functionality, and now it's been incorporated!