Only way to disable this is to open about:config and change some value. Why this has no dismiss button?
There is couple more settings that are on by default and i hate those; but mostly those are subjective. However who wants 80px long "SIGN IN SO WE STORE YOUR BOOKMARKS, HISTORY IN OUR SERVERS" stuff on their screen?
Right, but then its no longer exactly the same functionality, because it removes a useful functionality. Even if all Ai features are BS for you, its still a useful functionality that is being removed, and therefore no longer "exactly the same functionality". Because it is done offline on your machine and the results are pretty good from my experience. I use it often (translation). And there is no extension available integrated this well into the browser, doing the job this good and stays offline.