@hdv
"Make it a setting" sounds nice but no one considers the cost to users from having too many settings in UIs.
What a like about Firefox is people who really want to change all kinds of default behavior can do so through about:config and other means of altering those prefs.
The translation feature has been there for a while, is good for all the reasons you listed, and is fundamentally different from the LLMs, which are what people mean by "AI" today. Its pop-up panel already has a setting to stop it popping up any more and if someone doesn't even want to see the address bar icon, I'm sure there's something in about:config for that.