Hmmm. That’s suspicious, there’s a number of things in the way of video acceleration with that setup.
First of all, the fact that on fedora (ublue is a derivative of fedora) you need to install openh264 from dnf and not from Firefox extension manager, and then you still need to change some settings in about:config . Second, you are using a flatpak, I’m not sure if openh264 needs to be installed “inside the flatpak”. And last, it might just be the Nvidia.