Does it seem that the computer is struggling with the video, for example, the fan spins up loudly?

Could you check whether any updates are available for your display driver software? This article suggests safe ways to do that (random downloads from search engines are often malware):

Upgrade your graphics drivers to use hardware acceleration and WebGL

If no updates are available, try disabling Firefox's use of Hardware Acceleration to bypass potential incompatibilities with your display driver software. This article will get you to that setting, which takes effect the next time you exit Firefox and start it up again:

Firefox's performance settings

Any improvement?

Also, some video sites use Flash, and your Firefox reported the Flash plugin is version 28, which is quite a bit behind now. Please see: Flash Plugin - Keep it up to date and troubleshoot problems.