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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
Same way you test on Safari if you don't have a Mac, I guess. (i.e. not at all, or with the same rendering engine on a different device and hoping it is similar enough, or via a service like Browserstack.)