(I'm a program manager on Mozilla's video team, so I'm definitely interested in your YouTube problems. :) YouTube playback is common user complaint, but it can

(I'm a program manager on Mozilla's video team, so I'm definitely interested in your YouTube problems. :) YouTube playback is common user complaint, but it can be very configuration dependent.) When you say the browser freezes, is it permanently deadlocked or does the video eventually start playing? Firefox 38 on Linux does not support MSE (Media Source Extensions), so YouTube's HTML5 video player can't use adaptive streaming. YouTube is blocked on Firefox's download and rate estimation of the video. It sounds like the YouTube player or Firefox might be stuck in a busy loop downloading the video. To test whether the video download is causing the problem, you could try flipping the "media.mediasource.enabled" to about:config pref to true and "media.mediasource.whitelist" about:config pref to false (and restarting Firefox) to see if MSE helps. Alternately, MSE is enabled on Linux in Firefox Nightly (41) and Dev Edition (40) so you could just try them. Does Flash play without freezing, if you have Flash installed? Flash on Linux supports H.264, but Firefox only supports VP8, VP9, and WebM unless you have gstreamer's H.264 decoder installed (but it has its own problems).