Probably because there are other methods for Chrome that don't apply to Firefox.

Like when I noticed that some sites did some URL rewriting trickery on Firefox and others browsers, but not for Chrome. The trick is to show you the proper URL the link points to, but as you click, it is substituted for one that is a redirection, for tracking purposes (ex: "https://l.facebook.com/l.php?u=http:://actualsite..."). On Chrome, they don't need to use these tricks as the browser supports the "ping" attribute of links, so they can do their tracking without rewriting the URL.

I have no idea because I didn't experience anything like that both in Chrome and in Firefox (both with uBO though). But I'm confident that this particular code is not related to the actual slowdown, if it did happen to some Firefox users, because I received the same code even in Chrome.

Does Firefox allow a wider range of plugins, including adblockers?

This is just anecdote, but sometimes (especially when I'm on slower internet) Safari + AdGuard will have glitch [0] on YouTube. Never happened with Firefox + Ublock Origin.

[0] Unable to press play and showing image with Ad instead.

I would suspect because Google can do the detection in Chrome itself, but not in Firefox.