How do I download and install the extension in my iPhone

The Firefox web browser on iOS/iPadOS is basically a shell for webkit due to Apple's restrictions on what browser engine can be used. It does not use Gecko like the desktop and Android Firefox uses. https://support.mozilla.org/en-US/kb/add-ons-firefox-ios Extensions like these https://addons.mozilla.org/firefox/search/?q=MetaMask&type=extension are for desktop Firefox on Windows, macOS, Linux and Extensions for Android Firefox are at https://addons.mozilla.org/android/