I'm pretty sure this is about Mozilla services. AFAICT, Firefox itself is licensed under the https://de.m.wikipedia.org/wiki/Mozilla_Public_License and as such doesn't put any restrictions on how you use the software.
I think its a good argument for using a Firefox fork.
So either they're saying your use of Firefox, regardless of whether you want to use Mozilla services, must also follow the same acceptable use policy that your use of their services would, or it's a massively ambiguous way of saying your use of Firefox in combination with actual Mozilla services must comply with the policy.
If it's the former, their terms of use would be in conflict with the commonly understood definition of open source and free software licensing. If it's the latter, it's just poor legalese that fails to make its intent clear. (Interestingly, the Mozilla Public License does not seem to explicitly say that there are no restrictions regarding the use of the software for any particular purpose, although that is a commonly accepted part of the definition of free software and open source.)
[1] https://web.archive.org/web/20250228155328/https://www.mozil...
[2] https://www.mozilla.org/en-US/about/legal/acceptable-use/
> Mozilla software is made available to you under the terms of the Mozilla Public License 2, a free software license, which gives you the right to run the program for any purpose, to study how it works, to give copies to your friends and to modify it to meet your needs better. There is no separate End User License Agreement (EULA).