Rated 5 out of 5 stars
The very good idea, but we need an ability to toggle on/off this extension globally. And the toolbar button should visually show the on/off state of the extension. After that I give you 5+ points ;-)
Update: Ok, your extension is very useful. 5 stars! :-)
Some tips for the future: the toolbar button should display the status of the extension even when there are no previous pages to control the state from any page and a state of a browser. 2nd: I don't know why, but FireFox (3.6.23) reload previous pages instead of to load they from the cache.
i added a disabled icon for it, when you click the toolbar button then that should disable it globally