I can't say anything about Firefox, but if you're having performance issues and don't mind using a 3rd party youtube client, check out www/minitube or www/pipe-viewer (pipe-viewer recently imported a few days ago, you will have to backport if you're on -stable; I wrote this port and am using it on -stable so I know it works fine).
- Firefox vs Chromium youtube playback Mihai Popescu
- Re: Firefox vs Chromium youtube playback Stefan Moran