This is my favorite HN addon. I've used it on both Chrome and Firefox and it works like a CHAMP.

It adds a little settings icon to the top of HN, makes the website visually usable (since the default HN theme is terrible), and provides lots of useful utility functions like infinite scrolling, opening all links in a new tab, etc.

It's super easy to configure: just click the settings icon and toggle functionality on and off.

I honestly am not sure how anyone uses HN without this!

UPDATE (2023): After using this extension for several years, it appears to no longer be maintained. Much of the functionality has broken (infinite scroll, mark as read, etc.), which dramatically reduces the usefulness of the tool :(

My personal recommendation as of 2023 is to no longer use this.