120 fps web videos can only be viewed using a specific configuration of hardware and software. You must have a 120 Hz monitor as well as a recently released high powered GPU. Chrome, FireFox 24+, or IE 10+, work best to view 120 fps web videos. Some GPUs will only display HFR web videos on one specific web browser, so if you're having trouble with one browser try the most recent release of another. 

If you have the correct hardware and software but you're still unsure of your refresh rate, make sure your monitor is set to 120 Hz and visit Test UFO to determine your computer's capabilities.