As you might have heard already, Firebug has been discontinued as a separate Firefox add-on. The reason for this huge change is Electrolysis, Mozilla's project name for a redesign of Firefox architecture to improve responsiveness, stability, and security.

What replaced Firebug?

The Firebug web development tool, an open source add-on to the Firefox browser, is being discontinued after 12 years, replaced by Firefox Developer Tools.

What happened Firebug?

Firebug was discontinued last year (2017). So it's sad that Firebug is now reaching end-of-life in the Firefox browser, with the release of Firefox Quantum (version 57) next month. The good news is that all the capabilities of Firebug are now present in current Firefox Developer Tools.