I searched for a Firefox plug-in and found there wasn't one. I digged a little deeper and stumbled on this:

http://www.labnol.org/software/tutorials/keep-window-always-on-top/5213/

All you need to do is download and run the exe and press ctrl + space on any window (including Firefox windows) and that window will stick and always be on top of other windows. This is great it means I can catch up on TV and work at the same time!