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.
Its even more broad than that, because its any exchange of data for valuable consideration. No money has to change hands, but if it benefits FF, its a sale. And the benefit could simply be “if we do this we will function correctly as a browser”.