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

Use agent switcher plugin

https://addons.mozilla.org/en-US/firefox/addon/uaswitcher/

Spoofing the agent only makes it look like Firefox is being used less, and websites will care even less about it.

Change browser user-agent https://www.whatismybrowser.com/

Use extensions that change your user agent

You can find them in add-ons store

Find an extension to change your user agent