Extension Manager Extended
As you develop add-ons for Firefox, you'll notice that it's a pain to find the ID of the extension and where it was installed in your Firefox directory. The Extension Manager Extended add-on fixes all of that for you.
Install the add-on, restart Firefox, and then open the add-ons dialog under Tools. First thing you'll notice when you select an add-on is that two properties are added: an Install Date and the unique ID for the extension. Now, you don't need to worry about looking for the ID. But where is the add-on stored at?
Right-click on an add-on and notice two new menu items under the context menu: Copy ID and Open Containing Folder. Copy ID copies the ID to the clipboard and Open Containing Folder will take you directly to where the add-on is installed. Opening the containing folder is a definite help.
These four features alone are fantastic for Firefox add-on developers because it takes a commonly used dialog and extends it by including additional developer features. It's the little things that make us happy.
I definitely like this add-on and if you're a Firefox developer, this is a necessary add-on and appropriately deserves a rating of 5/5.