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.

As far as I can tell, it doesn't look like this - one of my favorite plugins - will survive v57.

See https://github.com/piroor/treestyletab/issues/1224

Thanks. I'd seen that thread months ago; it seems that the situation has improved a bit since then.

As I was searching around, I also came across this:

https://addons.mozilla.org/en-US/firefox/addon/tree-tabs/

...which looks to have similar functionality, and is already WE compatible.