Modification Rated 5 out of 5 stars
I have modified this addon, by making the icon look like Opera's Closed Tabs trash icon (which is like Undo Closed Tabs Button).
I have also removed that annoying small dropdown arrow to the right hand side of the button (as it is far too small to click on), and have made the button that when clicked on opens the menu showing recently closed tabs.
Here is a screenshot: http://i44.tinypic.com/152lykw.png
Here is the download link: http://www.mediafire.com/?ldvvek54v14sa03
After downloading to a specified location, just 'drag and drop' the file into firefox, it will ask to install after about 5 seconds.
If you cannot find where you downloaded your file, in the 'Downloads' list right click on the file, and select 'Open Containing Folder', then 'drag and drop'.
Note: Tested with Firefox 8.0!