Hi, Could you try in [https://support.mozilla.org/en-US/kb/diagnose-firefox-issues-using-troubleshoot-mode#w_how-to-start-firefox-in-4troubleshoot-modesf5safe-modesf Troubleshooting Mode] to see if you can reproduce the issue. This will make sure it isn't, for example, due to an addon installed in your Firefox. Also, could you try without Hardware Acceleration. You can find the instruction to turn it off at https://support.mozilla.org/kb/upgrade-graphics-drivers-use-hardware-acceleration In the last article I linked, there's also instructions on how to know if your graphic drivers are up-to-date. It may be also a good idea to look at it.

Indeed, when I activate troubleshotting mode, videos are displayed correctly. I tried disabling all extensions but the problem continued, so I tried disabling software acceleration and the problem was fixed. Thank you very much for your help.