It is very clearly mentioned in ToS that it applies to Firefox as well.

"You Are Responsible for the Consequences of Your Use of Firefox

Your use of Firefox must follow Mozilla’s Acceptable Use Policy, and you agree that you will not use Firefox to infringe anyone’s rights or violate any applicable laws or regulations.

You will not do anything that interferes with or disrupts Mozilla’s services or products (or the servers and networks which are connected to Mozilla’s services)."