Thank you very much for your insights, Matt and @nadb!
Short version: I’ve rectified the kernel issue, burned xorg-x11-drv-intel with fire, and removed the i915.enable_psr=0 parameter as advised. (I haven’t gone through the rpmfusion step yet, because I’d like to test this for a few days to see if it’s sufficient.)
Long version: I’m not entirely sure how my kernel ended up this way, because I used a freshly downloaded ISO file for my install last Saturday, but I had incorrectly understood that it was the latest version because GNOME Software indicated that my system was up to date. (So I assume this only indicates installation, and not active usage.)
In any case, I remembered seeing a list of something that looked like a list of kernel versions while rebooting the other day, so I rebooted, pressed F8, and saw that I could simply select the correct one (later made persistent):
Fedora Linux (6.5.7-200.fc38.x86_64) 38 (Workstation Edition)
Fedora Linux (6.5.6-200.fc38.x86_64) 38 (Workstation Edition)
*Fedora Linux (6.2.9-300.fc38.x86_64) 38 (Workstation Edition)
Fedora Linux (0-rescue-7db6c6aba4d84f0e939088d60f407bf8) 38 (Workstation Edition)
UEFI Firmware Settings
Also, to answer this question for later reference:
Apparently, the kernel being old and such had made it so that the regular Firefox could not play anything on YouTube. Therefore, I had dug up this solution. However, now that the root causes of the issue are resolved, I’m happy to report that the regular Firefox can indeed play YouTube videos without a problem. 