Without having much knowledge of AI models beyond surface level stuff I read, but a good understanding on how computers work it seems fairly predictable to me that running an AI model in the browser session locally could be kind of heavy on the CPU usage. As such you would think as a developer you would start with adding the feature as off by default, so users that want it can turn it on and you can get some real world metrics on how bad that hit is going to be before bending the entire userbase over the AI kitchen table.

So both doing it for something as trivial as tab grouping and making it something you have to go into about:config to disable seems really stupid.