downloads of videos from youtube do just the audio, no video!
Using Firefox, at youtube, I download a mp4 video,,it downloads,,,, but sometimes
it is just the VIDEO part of the file, sometimes it is just the AUDIO part! I never get
the "complete" file, with both audio and video!!
The Firefox 19 version that you currently run is no longer supported with security updates.
It is important to update Firefox and add-ons to the latest version to get all security fixes.
Start Firefox in Safe Mode to check if one of the extensions (Firefox/Firefox/Tools > Add-ons > Extensions) or if hardware acceleration is causing the problem (switch to the DEFAULT theme: Firefox/Firefox/Tools > Add-ons > Appearance).
Do NOT click the Reset button on the Safe Mode start window.