Step by step:
1. make the video stream play.
2. open the Blockable Items list. (ctrl+shift+v)
3. in the search bar, write: videoid
4. right click -> copy address.
5. close tab.
You now have the url for the file, usually you just paste the url into the addressbar, but depending on the filetype, in this case .mp4, firefox might not let you download it.
If you have Downthemall, you can force it to download the file with the Add Url button in its manager.