You  might have heard the news about availability of Firefox 4.0 beta through Mozilla FTP servers , Firefox 4.0 comes enabled with Tabs on Top much like to Google Chrome above the address bar, I have found yet another feature Firefox 4.0 shows undo button when you uninstall a particular extension.

Though Firefox users can install any extension by quickly going to Mozilla Add-ons page,  but it seems Firefox 4.0 has lot of developments under the hood, much like undo of extensions uninstall in Firefox 4.0.

undo for extensions uninstall  in firefox 4.0

So if you uninstall  an extension, Firefox 4.0 shows undo button which undoes the uninstall of particular extension.