Mozilla opposes a proposal because it goes against their principles and vision for the Web.
They believe that any browser, server, or publisher that follows common standards should be considered part of the Web.
Standards are designed to be independent of specific hardware or software, allowing for a wide range of devices, operating systems, and browsers to access the Web.
This diversity of choices promotes accessibility and overcomes personal obstacles.
Mechanisms that restrict these choices harm the openness of the Web and are not beneficial for users.

The proposal's use cases rely on the ability to detect non-human traffic, which could hinder assistive technologies, automatic testing, and archiving and search engine spiders.
These tools require access to content intended for humans in order to transform, test, index, and summarize it.
The proposed safeguards are unlikely to be effective and fail to address these concerns adequately.

Mozilla acknowledges the importance of addressing fraud and invalid traffic but finds the proposal lacking in practical progress for the listed use cases and highlights clear downsides to its adoption.

Credit: https://chatbot.theb.ai/ just another version of https://chat.openai.com/ without the need to sign-up.