µBlock - An efficient blocker for Chromium, Firefox, and Safari. Fast and lean. https://github.com/gorhill/uBlock
Chromium - You can install the latest version manually, from the Chrome Web Store, or from the Opera store.
Firefox - You can install by dragging the latest uBlock.firefox.xpi file to your add-on page.
Safari - You can get µBlock for Safari (Yosemite's 8.0 or newer) from the Safari Extension Gallery.