As most such video is flash, the idea is to play them not with the flash plugin, but with smplayer or vlc plugins.
Youtube is the most easy to play externally.
SMPlayer has an integrated Youtube Player (smtube) which has a search option.
But this is limited to youtube and the buffering has problems sometimes.
VLC behaves better and seems to be able to play other sources (at least Dailymotion, as far as I can tell). Copy video link and paste in Media - Open network stream.
To automate a bit this operation, add OpenWith addon to Firefox, go to its preferences and add VLC player there, and then set it to appear in context menu.
Then search something in Youtube or Dailymotion and in the list with findings right-click and open in VLC.