Regarding the High CPU Usage with firefox:
The Fedora mesa package dropped support for those codecs here
The folks at rpmfusion have built mesa vaapi drver package that supports these codecs :mesa-va-drivers-freeworld it’s available but still in the update-testing repository of rpmfusion. This is also referenced in your original post, Firefox using more CPU while playing videos - #14 by ozeszty
Regarding the OpenH264 addon in your screenshot, you should be able to enable that by selecting the 3 dots on the right and activate always.
https://docs.fedoraproject.org/en-US/quick-docs/openh264/