DragDropUpload is a Firefox extension which allows files to be dragged into form fields instead of using the Browse button. I think that's how file uploads should work; the Browse button is ridiculous.
Version 1.6.8 slows down Firefox to the point of unusability on some sites. Replacing dragdropupload.jar (in the corresponding extension directiory in the profile) with the one from 1.6.7 seems to fix this problem. (Yes, this means 1.6.7 is installed but Firefox thinks 1.6.8 is installed. It's exactly what I want. I don't want to be nagged to upgrade to 1.6.8.)