H.265 doesn't seem to be widely supported at the moment: https://caniuse.com/#feat=hevc Patents appear to be an issue that needs to be solved (or worked around) before it could be implemented in Firefox: https://blog.mozilla.org/blog/2018/07/11/royalty-free-web-video-codecs/ As you may know, Firefox doesn't natively support H.264 and instead uses third party libraries such as FFmpeg (on Linux) or Media Foundation (on Windows 7-10). I imagine there will be a similar strategy for H.265 if it is ever supported "in tab".