“Last, and tiniest, is dragdropupload. This little add-on is the
simplest of the three, but for me the most likely to stay added to
my copy of Firefox. Instead of browsing for the file you want to
attach to a message, dragdropupload lets you drag and drop the
file’s icon or file name onto the entry bar. This works best if you
keep a window open for the directory in which most of your
documents and other files reside. There are no options or settings
to configure with this add-on — simply install it and you’re ready
to save time, keystrokes, and mouseclicks.”