In this new post to his blog, Sameer shows how you can push content with the xpi content type out from your PHP scripts (for things like a Firefox toolbar/extension).

A couple of days back I created a Firefox toolbar for my blog as an experiment in learning XUL. Once installed on my blog I wanted Firefox to recognize it as an addon and install it rather than displaying a 'save/open' dialog.

Since his hosting provider (GoDaddy) didn't seem to work with adding it into an .htaccess file (an AddType for Apache), he had to force the download type with a few calls to header with the right content type, file size and file name to push it to the user.