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Mozilla is already revising its new Firefox terms to clarify how it handles user data
(www.theverge.com)
This is a most excellent place for technology news and articles.
I think this is a reasonable explanation.
But I also believe a large part of the firefox user base does not want any data about them collected by their browser, no matter if it is for commercial purposes or simply analytics / telemetry. Which is why the original statement "we will never sell any of your data" was just good enough for them, and anything mozilla is now saying is basically not good enough, no matter how much they clarify it to mean "not selling in the colloquial sense"