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, simply go to your Gmail, for example, and "compose" a new message. Hit the "add attachment" button, and instead of browsing through your complex file tree from there, you 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 you have as many attachment boxes open as files you'll be adding.
Dragdropupload supports a sidebar file tree navigator, but I wasn't able to get it to work. This didn't cause any big problems, though, since the Explorer tree navigation felt more natural than the unfamiliar sidebar nav. Not exactly feature-filled, this app does what it promises, and it doesn't cause Firefox to run any slower than any other plug-in would.
If you find that it doesn't work, trying installing in conjunction with a compatibility override tool such as Nightly Tester Tools or MR Tech Toolkit. Be warned that this might affect the stability of your browser, but I've had no problems so far.