Mozilla still has terms and conditions, so there's still a relationship, and still a liability for them letting a customer misuse their browser, even if they don't keep data on everyone.

While I absolutely agree it's ridiculous, as I read it, it would also apply to self-hosted software and things like thunderbird that are technically a browser.

Still, I expect enforcement to really only care about "real" browsers, not one user and their own thing or someone using Thunderbird to browse the web. France (and most other governments) have shown multiple times that they don't really look into the how they'd do these things before they try to make it law and it'd be a mess.

As per the article this post linked, this would definitely be a new precedent, browsers have never been responsible for this content, and whatever actually happens is up in the air. I'm mostly talking worst-case scenario. It's entirely possible some other business or consumer protection law makes this unenforceable, or any number of other situations, but since the French government decides how unreasonable they're gonna be, that's all up to them. Maybe they crusade against Firefox, maybe they give up when they realize there's only so much to do without drafting even more, and maybe they do go after everyone, including thunderbird or any other app that opens a webpage. Probably just ones that navigate to the illegal webpages though.

Still, a measure that's completely defeated by a VPN, unless they add all of them to their illegal pages.