console.log, but it should just be one of the many tools in your toolbelt.
If you want to learn more about Firefox Debugger, check out the Firefox Debugger Documentation on MDN.
Mozilla recently rebuilt Firefox Debugger from the ground-up using React and Redux. This new Debugger is designed to be approachable, yet powerful. And the best part? It is completely hackable. It is available right now in Firefox Quantum. Check out the repository on GitHub.
That's a wrap! If you haven't yet, check out Firefox Developer Edition. It has the latest features, is blazingly fast, and comes packed with all the development tools you need to build for the open web.