Solution: Use uBlock Origin, or User Agent switcher
Story via YouTube seemingly intentionally slow loading on Firefox while Google Chrome works fineThe latest such incident seems to be happening right now as there are several user reports online which suggest that YouTube is being intentionally crippled on Firefox and is being made to load up slower.
At first glance, if your internet connection is fine, it will seem like the issue could be related to performance and hardware optimization on Firefox's behalf, perhaps poor or sub-optimal hardware acceleration support or the improper utilization of the available processor threads, among other system resources. However, users found that the issue—a 5-second delay when loading a YouTube video on the Firefox browser—goes away when YouTube is tricked into thinking that it is running on Google Chrome using a User Agent switcher.
A Redditor paintboth1234 suggests the problem is due to a five-second delay, which is in the code.
Code:setTimeout(function() { c(); a.resolve(1) }, 5E3);
Apparently, the problem does not occur when uBlock Origin is used, so if you are annoyed by the five-second loading of YouTube on Firefox and don't want to use a User Agent switcher workaround, you can always use the good ol' uBlock Origin.