https://addons.mozilla.org/en-US/firefox/addon/video-downloadhelper/
This should be all that you need. I've tried a lot of addons for this sort of thing and this is the only one that I've had great results with.
Had a bit of a listen - This guy is really good. Really easy to listen to.
Just tried to download the foreword with the addon above and it worked perfectly.
here's a quick guide I found if you need further instruction.
https://www.youtube.com/watch?v=16UR1lzPoE0