I found an old port of Firefox but its support for more modern language features was lacking.
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There are new browsers coming, one of which is based on webkit, the details where released at the Wakefield riscos user show last weekend (April 27th 2019), I can't recall the name, but it's getting released to trade and investors first, and then a while later it will hit the public.
You can find the full report for the most recent show on "iconbar.com" :
(PS: If you look in the pictures from the show, I'm hovering around in some of them, I was down at the 8-bit end of the hall with ABug/Stardot and the rest of the BBC crowd) :-)
(PPS: You may also want to come and join the stardot forums: https://stardot.org.uk/forums/index.php pretty much anyone whos involved in the Acorn/BBC/Archimedies/RiscOs/rPI scene is a member there)