Excellent extension, thanks! Simple, useful, tasteful.

For those a bit confused as to what this extension does, there are four ways to use it:

1. Left-click on the toolbar button to re-open the last-closed tab.
2. Right-click on the toolbar button to see the last 6 closed tabs, or more in the "More entries" submenu.
3. Right-click on a tab and use the "Undo Close Tab" submenu on the context menu.
4. Right-click on a web page and use the "Undo Close Tab" submenu on the context menu.

I would love to see the ability to re-open closed windows. For now, it seems the only way to do this is to go through Open Menu > Library > History > Recently Closed Windows, which is about two clicks too many for my liking. But I understand you have already investigated it and it is not currently possible. Hope to see it as a future enhancement if Mozilla adds the relevant APIs.