> I think it'd be nice to have a browser option where it simply won't load content from outside of the current domain.

Take a look at uMatrix[0]. With the two rules

* * * block

* 1st-party * allow

you'll achieve what you want.

> To me what's more alarming is what percentage of websites load js from Google

That's where the Decentraleyes[1] plugin comes in handy. It injects a locally cached version of 3rd party cdn libraries. To use it easily in conjunction with uMatrix, you'll need some more rules that you can find in the FAQ[2].

[0]: https://addons.mozilla.org/en-US/firefox/addon/umatrix/

[1]: https://addons.mozilla.org/en-US/firefox/addon/decentraleyes...

[2]: https://github.com/Synzvato/decentraleyes/wiki/Frequently-As...