This is a simple extension designed to open up your default mail client's compose window with an attachment of the current page your viewing in Firefox already added. It is designed to support Thunderbird and Outlook (sorry Outlook Express users your going to have to get Thunderbird if you want this to work). Other email programs may work, and I would be interested to know which ones do. Once installed the extension can be either used from a menu item in the "Tools" menu, or from a toolbar button that you will find in the customize toolbar window.
The extension comes with an option window that contains two options. The first one is the path to your email client. This should be auto filled on Windows. And then the application your using. The options are Thunderbird, Outlook or Other. If "Other" does not work, you could try using Thunderbird as that might work instead.
I do plan to make the extension work a bit more smoothly in time, just I lack the time to do so right now.