@KimsCrochets Well this could very well be a case of of old technology meets new technology. And old usually looses out. Albeit it iOS released back in March of 2016, 2 1/2 years ago.
I would recommend you just add another browser, my suggestion would be Chrome but I found this discussion regarding that issue https://discussions.apple.com/thread/8458965 it sounds like you might not be able to do so unless you can upgrade your iOS (if that is possible).
Checking on Firefox for iOS it appears iOS 10.x is the oldest it will run on as well.
And after looking to see if you can upgrade your iPad 3 to at least iOS 10 and that doesn’t seem to be possible at least based on this page: https://everyi.com/by-capability/maximum-supported-ios-version-for-ipod-iphone-ipad.html
So not sure how far back in technology the Gutenberg team wants to support old technology, especially if very few users us that specific iOS and or Hardware like iPad3.
Might be time to look into upgrading to something newer.
Sorry I personally couldn’t be of further help.