Perhaps the biggest change in Firefox 15 is that it ships with a debugger that developers can use to find issues with their JavaScript code. The debugger is available via the Web Developer menu.

Mozilla began migrating Firebug features to the built-in Firefox debugging tools in 2016. Core Firefox developer tools include a page inspector, web console, JavaScript debugger, and network monitor.

Mozilla has launched Firefox 77 with faster JavaScript debugging, optional permissions for extensions, and Pocket recommendations in the U.K.

Debugger.html currently debugs Firefox, connecting via the Firefox Remote Debugging Protocol. It also can connect in an experimental fashion to the Chrome browser to debug Chrome tabs, and to Node ...

Each browser has its own remote debugging protocol, and the add-on translates from those "foreign" protocols back to Firefox's own built-in remote debugging protocol.

Whether you're a Web designer or a JavaScript programmer, Firebug provides invaluable tools for debugging websites.