"Better Private browsing" that still features the Pocket bundleware and button in the toolbar that's still not un-installable like other extensions and requires about:config edits to disable..
But it doesn't do anything unless you opt-in very strongly.
/flamewar
And space-wise we're talking a couple kilobytes.
You can't really have an opt-in without a point at which you ask the user about the option. The easily-removed button does that, the fact that it's not truly purged from the browser is a geeky detail that they're working on fixing.