Leveraging pdf.js in Thunderbird
Thunderbird Conversations 2.2.1 has been released.
You can now view PDF attachments inside Thunderbird using pdf.js! You don't need an external viewer anymore. That's the pdf.js magic.
Clicking the icon opens a new tab with a mini-viewer (very basic functionality). It works pretty well, and the plan is to add a viewer inside the conversation, so that you can get some sort of inline pdf viewer, just below the email. That's a reasonably sized chunk of work, but I'm really busy right now, so it's not going to happen anytime soon; therefore, anyone coming with a patch would be extremely well-received :).