Software running on my machine is not a "service", so it does not need a "Terms of Service".

Firefox Sync needs a ToS. Firefox Relay (the email masking thing) needs a ToS. Firefox web browser does not.

There is no need for a ToS unless your software has privacy issues because you are spying on users without their 100% informed consent. There is a lot of software that does though.

https://wiki.debian.org/PrivacyIssues

The only ToS of a free software should be the chosen free software license (and if applicable, a trademark policy). Anything else is questionable.

Does it? How much other OSS has a Terms of Use? I can understand it for firefox services like accounts, but for just using the browser I don't see how this makes sense.