Here are a couple of such add-ons / extensions: User-Agent Switcher https://addons.mozilla.org/en-US/firefox/addon/uaswitcher/ https://addons.mozilla.org/en-US

Here are a couple of such add-ons / extensions: User-Agent Switcher https://addons.mozilla.org/en-US/firefox/addon/uaswitcher/ https://addons.mozilla.org/en-US/firefox/addon/uaswitcher/ Custom UserAgent String https://addons.mozilla.org/en-US/firefox/addon/custom-user-agent-revived/ 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-string-switcher/ 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-a-firefox-extension-that-lets-you-set-a-user-agent-on-a-per-site-basis/#comment-4447842 https://www.ghacks.net/2019/12/23/custom-useragent-string-is...