This doesn't happen on all videos, it seems to be more of the lower quality videos than anything but still quite annoying. Only way I can describe the problem is that the colours shift about 100 pixels to the right in fullscreen mode, any help is greatly apperciated.
Anyway here are comparison screenshots of this video youtube.com/watch?v=UPHuE5pDlEs
My specs and software versions:
Adobe Flash Player 11.5.502.146
Windows 64-bit SP1, English Firefox 18.0.1 (Only enabled plugin is Flash)
CPU: Intel Xeon E3-1230v2
Graphics: Nvidia GT220 (Driver version 310.90)