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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 have not dug too deep into it for now (especially if I end up changing browser), but even with everything in the preferences disabled, examining the content of about:config gives a lot of telemetry.whatever.enabled left to true, sometimes with names that do not seem to match any option given to the user. That's not a good look either.
And you cannot change those in the default mobile Firefox since about:config is disabled (by their claim that it may break stuff in the ui)