Hi. There is a downloadable software which name is "downloadhelper" and which is very easy to find on the net. Once downloaded and installed on your computer, it shows an icon in the firefox toolbar (I'm not sure it works with ie).

When reading a vid you want to download, just click on this icon, chose the vid you want to save (usually the last one of the list, in bold letters), the place you want to save it and here it goes.

Works with Youtube, Dailymotion, Xtube and many others...