Sometimes, an older App-version can be extracted from the App Store. This isn’t guaranteed to work, but if an “old” version of an App is still on the App Store servers its always worth a try...
Assuming you have a “newer” model of iPad availbale to you (registered to the same AppleID) find and install the wanted App (in this case - Firefox) on the “newer” iPad. Now, return to the App Store on your older iPad, and from your Purchase History, look for the App (again, in this example - Firefox); If available, touch the cloud icon to download.
If you don’t want or need the targetted App on your newer device - simply delete it; the App will remain in your purchase history should you need to re-download at a later time.
This mechanism only works for App versions that have been withdrawn from sale (this includes “free” Apps) but remain available to re-download. If an App/version has been both withdrawn and removed from the App Store Servers, this mechanism won’t work.
If the old/new devices are registered to different AppleIDs, the iOS Family Sharing feature can come to your rescue.
Hope this helps - it’s at least worth a try.