Perhaps get a newer device? Support ended for iOS 10.3 3 years ago, and devices stuck on it would be up to 8 years old. Fairly common for devices of that age to not fully support some newer technology. Most likely something tied into 64 bit, but Firefox have said since last year that the oldest supported iOS is 11
As above, most likely the Firefox services removed support for TLS 1.0 and 1.1 and your browser doesn't support services above it.
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I am curious: Why are you using such an old iOS version?