Restore the mail commands in Firefox
From MozillaZine Knowledge Base
In Firefox 2, the mail integration toolbar and menu items have been removed. See the related bug reports listed below and this MozillaZine forum thread about problems with the original mail command.
To restore the removed mail integration features, install the Get Mail extension. It adds Tools -> Get Mail -> Read Mail, Tools -> Get Mail -> New Mail, and an optional "Get Mail" button on the toolbar (after installing, right click on the toolbar , select "Customize" , drag and drop the button to the navigation toolbar , and press "Done".)
Related bug reports