Hello Mozilla, If you are trying to manually update your existing Mozilla Firefox on iOS to the latest version, follow the instructions on this link from Apple. https://support.apple.com/en-my/HT202180 If you are trying to download the latest version of Mozilla Firefox from the App Store, just searching for "Mozilla Firefox" and installing it will get you the latest version. Hopefully this helps, and if it does, kindly mark this answer as the solution (solved)! If it did not help, kindly report back!

https://support.mozilla.org/en-US/kb/install-firefox-your-ipad-iphone-or-ipod