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Deciding for ourselves: 98% of people want a browser choice screen, Mozilla study finds
(blog.mozilla.org)
A place to discuss the news and latest developments on the open-source browser Firefox
From the full report:
So given that they used third-party providers, I don't think they would have been biased to Firefox users specifically. (And in fact, given the current state of the market, the majority probably wasn't a Firefox user.)