Youtube has started to artificially slow down video load times if you use Firefox. Spoofing Chrome magically makes this problem go away.

You can change your user agent string, the text your browser uses to tell the web site you're looking at what browser it is, either via your F12 developer tools menu or via an extension.

The most convenient way is with a browser extension that changes your user agent. You can also change it in the developer options of most browsers.