You probably don’t have hardware acceleration activated. Please provide us with your hardware information.
With AMD GPUs you can enable hardware acceleration according to the Arch Wiki.
I personally have a user.js file, that enables hardware accerlation in my current Firefox profile. Create a file user.js and copy it to your current used Firefox profile folder in ~/.mozilla/firefox/. Usually your current profile has the string release inside it. This should be the content of the user.jsfile:
user_pref(“gfx.webrender.all”, true);
user_pref(“media.ffmpeg.vaapi.enabled”, true);
user_pref(“media.ffvpx.enabled”, false);
user_pref(“media.rdd-vpx.enabled”, false);
user_pref(“media.navigator.mediadatadecoder_vpx_enabled”, true);
Those lines are the options that need to be set, according to the arch wiki.
Also, if you use X11, you need to set the environment variable like this:
MOZ_X11_EGL=1
For Wayland like this:
MOZ_ENABLE_WAYLAND=1
All of this assumes you have an AMD GPU with VA-API support, according to the wiki, and you also need to install the libva-mesa-driver package. Maybe this might help some others, too.
For Nvidia GPUs, which only support VD-PAU outside of the open source Nouveau driver, there seems to be a translation layer. But I don’t have any experience with that, so if you have an Nvidia GPU, we just need to see if that works. For Intel’s integrated graphics, the mentioned user.js will work, but you need a different VA-API driver package (see the wiki).
If you have questions and problems along the way, just ask them!