I don’t understand how they are supposed to “sell your data” if you just never use a Mozilla account and uncheck all the telemetry. Its not like they can secretly steal your data, since its Open Source.
It seems to me like just more FUD that Google is spreading to undermine our trust in free software.
The problem is that a reduction in trust correlates to a reduction in users. A reduction in Firefox increases Chromium’s dominance on the web, which is a near monopoly already. A monopoly on web renderers in turn is bad for open web standards.