Hi I need your help User-Agent Switcher and Manager

If you mean this Extension https://addons.mozilla.org/firefox/addon/user-agent-string-switcher/ then it will not work on Firefox for iOS/iPadOS. Due to Apple's restrictions on what browser engine can be used on iPad/iPhone the Firefox web browser there are currently no Extensions for Firefox on iOS/iPadOS. https://support.mozilla.org/en-US/kb/add-ons-firefox-ios The Extensions for Firefox at https://addons.mozilla.org/firefox/ is for desktop Firefox (Windows, macOS, Linux) while the Extensions at https://addons.mozilla.org/android/ are for Firefox for Android.