I use greasemonkey scripts:
https://gist.github.com/kennethrapp/7a21c0187fedd6f47e7c
highlights lines beginning with ">", which helps differentiating quoted replies from comment text. And yes, the idea comes from imageboards, but a lot of people on HN use that syntax in their comments.
https://gist.github.com/kennethrapp/5b5e413220afb93c9c93
removes fading from downvoted and dead comments and text posts.
On Safari I use hckr news[0]. It highlights new comments in threads I've already visited, and allows collapsing. There is also the option use the homepage there, but I just use the normal one.
I am a fan of Gerogify which focuses on the typography of the site. I think it makes it a lot more readable.
https://chrome.google.com/webstore/detail/georgify/ofjfdfale...
Hacker News Room http://hackernewsroom.com is a web-based HN reader, built on the official HN API. It's not a browser extension, but it has some nice features including folding comments. And it works well in all modern browsers.
Disclaimer: I built it.
Collapse comments is pretty light and makes reading comments easier.
https://chrome.google.com/webstore/detail/hacker-news-collap...
If Chrome, you may be thinking about HackerNew: https://chrome.google.com/webstore/detail/hackernew/lgoghlnd...
HN Utility Suite on firefox: https://addons.mozilla.org/en-us/firefox/addon/hn-utility-su...
Hacker News: Mark All Read [1]
Chrome works good.
StyleBot for Chrome to adjust the width.
Hacker News Enhancement Suite for Chrome
https://chrome.google.com/webstore/detail/hacker-news-enhanc...