"You can check if your browser supports all the codecs for YouTube by checking the YouTube HTML5 Video Player website, which list everything supported on your PC. If you have a Linux systems and Firefox 40, it's likely that some of those codecs will have a red sign, meaning that they are not enabled. All you have to do is open about:config and make the following changes. Please make sure that you don't change anything else. ..."

Source: Softpedia
 How to Enable 1080p YouTube Support in Firefox 40 for Linux