The privacy centric way for Mozilla to have address this would have been to:

• acknowledge laws in certain countries have changed
• Due to those new laws, the definition of “sell” has changed and Firefox may no longer be in compliance with their desire to keep your data private
• Commit their desire to take the necessary steps to keep new versions of Firefox in line with their original vision
• update the “we will not sell” definition to within the jurisdiction of the United States, or indicate that the definition of sell may be different in different jurisdictions
• make the necessary extensions to jurisdictions where they were “selling” user data, self reporting where necessary