The ToS mentions you have to follow their Acceptable Use policy, found here[1]. I guess that's what OP refers to, as it contains the following:

Upload, download, transmit, display, or grant access to content that includes graphic depictions of sexuality or violence,

However, the Acceptable Use Policy only covers Mozilla's services, as mentioned at the very start of the document and further down. So I don't see how it covers the browser itself, as it is not a service.

[1]: https://www.mozilla.org/en-US/about/legal/acceptable-use/