In 2004, our very own DamageInc created the first 4chan extension as a Firefox add-on. Now, we've implemented these features directly into the site, open sourced it, and invite the community to contribute code and feedback.
In addition, we offer a read-only JSON API that you can read more about here.
Bugs and feature requests should be submitted to the repo's Issues page.
This code is released under a 3-clause BSD license, available here.
Many thanks to desuwa of The Yotsuba Catalog for being a total badass, and almost single-handedly bringing this project to fruition.