I use preinstalled Fedora’s Firefox and I had an issue with videos freezing when jumping to a timestamp.

Clicking on a timestamp on comments or with the mouse on the time would make the video become a still frame with the audio still working.

The issue was Fedora’s “free” (open source) version of ffmpeg: I have installed the version from RPM fusion and it works well!

Link to RPM Fusion: https://rpmfusion.org/Howto/Multimedia