Firefox for iOS (for iPhone/iPad) currently requires iOS 14.0 or later to install and run.

In the past, I've updated Firefox for iOS using Apple's AppStore app that came with my iPad. It gives me a list of all the apps that need updating and I tell it which ones to update.

Currently, it seems that list has moved into the account popup (tap the little person-in-circle icon on the right side to call that up).