Unobtrusive Firefox Plugin Click-to-Install

I've been working on a really cool new project, to be announced soon, where I've built a Rails-based web app that has two interfaces: one for humans to interact with inside of a browser, and a RESTful API for browser plugins to interact with via GETs and POSTs.  We want people to be able to interact with our site while visiting other sites.

I had seen other Firefox plugins that were click-to-install, but I had a hard time figuring out how to make it work for our plugin. Firefox users were having to "Save Link As.." and open the downloaded .xpi file manually.  Very old-fashioned.  So here's a quick note to help future Mozilla or Firefox developers who need to create a click-to-install plugin:

1) It's all done through Javascript, so anyone without Javascript will have to install your plugin the old-fashioned way.  The Mozilla site documents the API call you need to make.

2) I'm a huge proponent of unobtrusive javascript (UJS), which I learned by using Dan Webb's excellent LowPro framework. Thus I wanted to make sure that the click-to-install javascript was offered as a progressive enhancement to the normal HTML links we provided.  That way, everyone could have a link to the plugin file itself for manual installation, but people with Javascript could enjoy click-to-install.

In this part of the site, we weren't using any other Javascript libraries, so it seemed like overkill to include Prototype and LowPro just for this one effect.  So it was a great chance to learn how to roll my own UJS without library support.  I whipped up a quick UJS click-to-install technique following inspiration from this presentation.  Here's what I came up with:

<script defer="defer" type="text/javascript">
//<![CDATA[
function doXPITrigger() {
if (!document.getElementsByTagName) return false;
var links = document.getElementsByTagName("a");
for (var i=0; i < links.length; i++) {
 if (links[i].className.match("firefox")) {
   links[i].onclick = function() {
  xpi={'Awesome New Project Toolbar':'/downloads/awesome_project.xpi'};
  InstallTrigger.install(xpi);
     return false;
   }
 }
}
}

window.onload = doXPITrigger
//]]>

3) I've seen other advice recommending you configure your web server to recognize the .xpi mimetype appropriately.  I did this but it didn't make much difference in my case.  Still, it's probably worth doing.  I added this line to our Apache config:

AddType application/x-xpinstall .xpi