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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
Interesting question. If a binary is available you can sideload already, you'd have to put the phone in Developer Mode and use either XCode or one of the 3rd party tools for macos or Windows to install it. Main question is how easy it'd be to find a trustable official Mozilla binary.