I addressed this preemptively earlier today [1]. As it happens, I would also like to enable or disable new tab pages without disabling an extension entirely. Perhaps you could file an issue with Mozilla asking for this functionality? I'd integrate it into Tridactyl in an instant if I could.

Regarding the focus stealing: having focus on the page was one of our most requested features. Unfortunately it isn't trivial to have it be optional.

[1]: https://news.ycombinator.com/item?id=24615963