Think that every conceivable useful feature not included in Firefox has been created? Not likely. At least now, though, we can take attachment drag and drop off the list with Dragdropupload.
The add-on works with most major Web mail services and message board systems. Once installed, users simply go to Gmail, for example, and "compose" a new message. Hit the "add attachment" button, and instead of browsing through the complex file tree from there, users can grab the file from Windows Explorer and drag it over the file location box in the page. The plug-in supports multiple files, too, as long as the number of attachment boxes open matchs the number of files to upload.
Dragdropupload supports a sidebar file tree navigator, but I wasn't able to get it to work. This didn't cause me any big problems, though, since the Explorer tree navigation felt more natural than the unfamiliar sidebar navigation.