Any project can insert any code they want, that’s how software works. The thing about open source projects like Firefox, is if they were to do such a thing, anyone could see it. You’re complaining about Firefox when they haven’t really done anything, and yet browsers like Brave have actually been caught inserting redirects into their links, which is harder to spot because Brave is not open source.

If you really agree this distrusting of Firefox, just don’t use it, or build it yourself.