HTML5 video lags in web browser

I have an nVidia GeForce 9500 GT on a 32-bit Linux Mate 17.2 system. If I try to watch 1080p videos in a web browser, the videos are lagging, even when they're fully buffered. If I download the video and play it outside of a browser, it works fine, and if I play it within a browser on Windows it's also fine. The graphics card driver I have on Linux is the best one, and installed correctly:
http://i58.tinypic.com/juii4k.png
http://i58.tinypic.com/30dfif7.png
If I run this Linux via Live USB and go to Youtube in Firefox, I see the videos play in Flash player and 1080p works fine. If I try to play HTML5 video in a media player, it lags. How can I fix this?