Use agent switcher plugin

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

Find an extension to change your user agent

deleted by creator

Use extensions that change your user agent

You can find them in add-ons store

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

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