Bing で次の結果が見つかりました
User-Agent Switcher – Firefox (ja) 向け拡張機能を入手 - Mozilla
User-Agent Switcher – Get this Extension for Firefox (en-US)
User-Agent Switcher – Firefox (ja) 向け拡張機能を入手 - Mozilla
Firefox拡張機能「User-Agent Switcher」の有効な使い方
How To Change the User Agent String in Firefox
How to reset the default user agent on Firefox | Firefox Help
GitHub - ray-lothian/UserAgent-Switcher: A User-Agent ...
How to Change User-Agents in Chrome, Edge, Safari & Firefox
手軽に「Firefox」のUserAgentを変更できる拡張機能「User ...
Firefox User Agent Switcher - how do I get to it? - Stack Overflow