I suggest trying the Download Helper add-on for Firefox.
When installed it shows up as a little colour animated icon when on a page with video that can be downloaded. Just click the down arrow to the right of it and select the resolution to download as. The ‘Medium’ option is usually 480p:
Unlike other downloaders, the MP4 file will generally play fine in any media player, such as VLC, Windows Media Player, MP4-capable TV, etc.