It's difficult to pick a single one.
uBlock Origin[0] is probably the one with biggest impact. I assume most of the HN folks know it, but It's a general purpose blocker that can block network requests and DOM content with support for AdBlock plus rules and host files. FilterLists[1] is a nice resource to find lists you can subscribe to.
I also like Privacy Settings[2] a lot. It's a GUI for changing all the built in features Firefox already has, allowing you to quickly switch options on and off as you like.