I use uBlock Origin to change the color scheme, remove visibility of my karma and filter out some submissions. I use FoxReplace in Firefox to replace words. I use the Canonical addon if I am submitting a lot of links though I still end up with dupes. I use Force Reader View for some sites that people submit to HN that require javascript to read as it sometimes gets around that requirement and makes some blogs more readable for me. The rest of the addons I use are not specific to HN.
I use a custom browser extension that is mainly used for filtering out pet peeves and regex matches and was wondering what everybody else used (if they use anything at all?).
I used to run a user script to hide karma numbers. [update: thanks Bender, since I run uBlock Origin I added a filter for karma again just now]