From the title of the article you are probably thinking that this is an extension that will add a feature to undo recently closed tabs. There’s no need for that because Firefox 2 already has that built-into the browser. The problem is that the feature isn’t very configurable and there is no easy way to quickly retrieve closed tabs.
The Undo Closed Tabs Button extension does a lot of things that Firefox should do by default. My favorite feature is that it will assign the keyboard shortcut Alt+Z to the undo function so that a quick press of those keys will open up your last closed tab. Firefox already uses the Ctrl+Z shortcut to undo other things which is why this extension uses Alt+Z.
It additionally has several other features that may interest you. For example, instead of having to go to the History Menu in order to get a listing of your “Recently Closed Tabs” this extension will also offer the ability to place a button right on your toolbar. In the options you can then choose how many closed tabs you want Firefox to remember but you probably don’t need to go beyond the default which is 10.
This is far from an extravagant extension but it is undoubtedly a time saver for me. I don’t think I even need to put this one in the “testing for usefulness” category (see the sidebar) because this one will be with me for a long time!