Jonathan Watt
As of bug 1536174 , I've added a mechanism for Firefox that uses an internal mechanism that can record content frames during composition. Note that this currently only works when on Windows using D3D11 composition. This is still in very early stages and will likely be getting some improvements over the longer term, but right now, basically this is how it works:
You will now find a new directory in your working directory called windowrecording-<timestamp> which contains a list of PNGs, this list of PNGs uses the following naming convention:
frame-<framenumber>-<ms since recording start>.png
Jonathan Watt