Video GPU acceleration but not the browser

[zini]

I want to disable Firefox GPU acceleration in general which I can do in the settings but enable video GPU acceleration e.g. in html5 video streams. The reason is because I run other GPU applictions in background the browser in general lags when hw acceleration is on globally - so I turned it off. But that lag doesn't matter for htmlt5 videos and it would reduce CPU usage if video could still use gpu hw acceleration. How can I do this? Are there any about:config settings which I can set?