I hope not. I believe that it is better for Firefox to be as light and bloat-free by default while users are free to install whatever addon provides any extra features that they need.
Tree Style Tab could be an opt-in functionality of vanilla Firefox, that wouldn't be an issue for new users.
Although I firmly believed that tabs on the side make more sense for any kind of users. Nesting tabs require a bit more practice though and that could be an opt-in option.
