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.
As someone else said, reproducible builds is a great mitigating factor for this secret changes. Firefox does have telemetry, but is very transparent and lets you turn it all off (as far as I can tell anyway). Don’t want ads? Easy. Don’t want Mozilla services? Simple.