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Apple is finally allowing Firefox to use its own engine on the iPhone (but only in the EU)
(www.theverge.com)
A place to discuss the news and latest developments on the open-source browser Firefox
Didn't even think about that haha.
I guess the best you could realistically do would be to adhere to web standards (not Chrome standards) and use desktop Firefox or Firefox on Android for testing as they should be the same internally as the hypothetical iOS port.