Bug 401272 made AMO send Thunderbird and Sunbird extensions with the "content-disposition: attachment" http header, see https://addons.mozilla.org/en-US/thunderbird. When clicking the "Download Now" link, Firefox asks: "What should Minefield do with this file? Open with/Save File? Do this automatically for files like this from now on?" But Firefox shouldn't ask, but save xpi files (Content-Type: application/x-xpinstall ) right away (when sent with "content-disposition: attachment"), as if I had chosen "Save File" and selected the "do this automatically checkbox" already. This should be possible without breaking xpi install in Firefox itself.