Comment by miles

Here are a couple of such add-ons / extensions:

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

Custom UserAgent String https://addons.mozilla.org/en-US/firefox/addon/custom-user-a...

User-Agent Switcher and Manager https://addons.mozilla.org/en-US/firefox/addon/user-agent-st...

and a comment by "99" pointing to an extension-free method to quickly change the user agent:

https://www.ghacks.net/2019/12/23/custom-useragent-string-is...