As a cross-platform solution for this, just use the Firefox addon FlashGot, which is meant to send video streams to different download managers. What if it were used to send the URL stream to VLC, for example?
- add VLC to Flashgot list: FlashGot options, General tab, Add, give a name 'VLC', - then go to the VLC executable and select it
- FlashGot options, FlashGot tab, choose VLC
- playing the video in Firefox, a Flashgot button should appear (add that to the toolbar or the status bar). Right-click the button, Available formats, and left-click the video. VLC should start playing the video stream
UPDATE:
After more testing mpv seems better than VLC for this purpose.
Other players can also be used in this way (in Windows, PotPlayer, MPC-HC, BSPlayer).
Also in Firefox, OpenWith addon.
Drag & drop:
On Youtube, after searching a term and getting a list of titles, dragging&dropping a name onto a player may start playing the video.
Not all video players work like this, and also late 2016 Firefox and Chrome doesn't seem to work in this way in Linux. But testing in Linux, it works with Seamonkey, Palemoon, Epiphany or Icecat browsers by dropping the title from Youtube in mpv and other front-ends for mpv like Gnome MPV and SMPlayer.
In Windows, it works in both Firefox and Chrome by dropping the title onto MPC-HC.