Thanks !
Solve a big problem with Waterfox classic + Youtube 360° and Nivida driver:
Youtube video seems to crash Waterfox content proces [Github].
Solution:
Install "enhanced-h264ify":
https://addons.mozilla.org/en-US/firefox/addon/enhanced-h264ify/
Open about:config:
1 - Set media.windows-media-foundation.allow-d3d11-dxva:false
2 - reset media.hardware-video-decoding.failed to false