Firefox for iOS 12.0
How do I install Firefox for iOS 12.5.6
How do I install Firefox for iOS 12.5.6
ప్రత్యుత్తరాలన్నీ (1)
Firefox requires iOS 13.0 at minimum.
You may be able to install older versions of a app if you had installed it in past and is in your purchase history. Not sure what the last version that sill worked on iOS 12. You cannot side load apps on it like you may be able to do with apk or Android.