Youtube does html5 by default and falls back to Flash Player. Check https://www.youtube.com/html5 to see what it says. Ignore the outdated table on http://www.quirksmode.org/html5/tests/video.html and scroll down to H.264/MP4. If the H.264/MP4 example does not work then you may not have the needed packages installed like Gstreamer and ffmpeg. While I actually do use Linux unlike Fred who is a Windows user, I cannot say what packages could indeed be the culprits outright in causing the crashes. Checking for updates to the packages mentioned in red in the crash reports in package manager is a start. Raspberry pi users do not post here that much to know of usual causes of crashes on such devices.