I generally agree with the sentiment—if something is free, there’s often a tradeoff. But when there’s a paid tier, the free version can act more like an entry point or hook to get users into the ecosystem, rather than relying on harvesting user data. In JetBrains’ case, broad adoption brings a lot of strategic value on its own (like establishing industry standards or building community mindshare), so it makes sense for them to offer a genuinely free version without necessarily treating users as the product.
This AUP is related to the optional backend services run by Mozilla. The frontends for all of these services are open source, with no usage restrictions. It doesn't affect how Firefox is licensed in terms of copyright. Additionally, the backends for some of these services (such as Firefox Sync and Mozilla Accounts) are fully open source, so you could avoid the AUP if you wanted to.