Cannot download to iPhone 17 pro max

If you go directly to https://www.firefox.com/download/ios/ there is a button that opens App Store for you. I see you're using Firefox for iOS to post this question so it looks like you managed to install and update the app just fine — unless you're having issues with another device? Any downloads and updates are handled in Apple's own App Store app, so you don't need to look for any similar options inside the browser, and can just close the download page after it took you to the App Store listing.