Thanks! Works for me perfectly in Zen!
In Zen: select Manage Extension → Cog → Manage Extension Shortcuts

This extension should never have been posted in this website, as it'll never work for both Firefox and Chrome/ium browsers. The extension container is for Firefox, but its functionality is for Chome/ium.

Doesn't seem to do anything in the latest version of waterfox, which is an open source browser derived from firefox that's compatible with 99.9% of firefox's addons. It's also very odd that this addon is repeatedly listed as being a chrome addon, yet was uploaded here as a firefox addon. This was almost certainly a mistake on OP's part.