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. Electrolysis’s multiprocess architecture makes it possible for Firefox to run its user interface (things like the address bar, the tabs and menus) in one process while the content (websites) runs in other processes. With multiprocess architecture, if a website crashes, it doesn’t also crash the whole browser.

Unfortunately, Firebug wasn’t designed with multiprocess in mind, and making it work in this new scenario would have required an extremely difficult and costly rewrite. The Firebug Working Group agreed they didn’t have enough resources to implement such a massive architectural change. Additionally, Firefox’s built-in developer tools have been gaining speed, so it made sense to base the next version of Firebug on these tools instead.

The decision was made that the next version of Firebug (codenamed Firebug.next) would build on top of Firefox DevTools, and Firebug would be merged into the built-in tools.

And perhaps most importantly, we joined forces to build the best developer tools together, rather than compete with each other. Many of Firebug’s core developers are on the DevTools team, including Jan ‘Honza’ Odvarko and Mike Ratcliffe. Other Firebug Working Group members like Sebastian Zartner and Florent Fayolle are also active DevTools contributors.

A huge thank you to them for bringing their expertise in browser developer tooling to the project!

In practical terms, what does it mean to merge Firebug into DevTools?

Several features have been absorbed: The DOM panel, the Firebug theme, Server-side log messages, the HTTP inspector (aka XHR Spy), and various popular add-ons like FireQuery, HAR export, and PixelPerfect. Also, over 40 bugs were fixed to close the gap between DevTools and Firebug.

For curious readers, a couple of articles on hacks.mozilla.org and on the Firebug blog go into more detail.

If you are switching now from Firebug to Firefox DevTools, you will of course notice differences. This migration guide can help you.

We understand that disruption is never really welcome, but we are working hard to ensure developers have the best possible tools, and sometimes this means we need to refocus and use resources wisely.

You can help: Tell us which features you need are missing. There are a few ways you can do this:

• Comment on this thread on the Mozilla dev-developer-tools mailing list.
• Share your feedback on this thread on the Firebug Google group.
• Or, post to this discussion thread on Twitter.

We are already tracking missing features on this bug, and so far you have told us that the most important are these:

• Break on XHR (bug 821610)
• Break on DOM mutations (bug 1004678)
• Better CSS auto-completion (like bug 1106336 and others)
• Various console auto-complete improvements: (bug 1267140, bug 1270015, bug 672733, and more)
• An events sidebar panel: (bug 1226640)
• Live previewing changes made in the inspector (when changing attributes or editing as HTML: bug 815464)
• Improvements to the way console log messages are displayed: (bug 1032855, bug 1165010 and more)
• Validating CSS values and selectors as you type: (bug 1227054)
• A DOM properties sidebar panel (bug 704094)
• Font-size changes in the Firebug theme (bug 1319079)
• An option to add cookies: (bug 1231451 and bug 1231452)

We thank you for your loyalty and hope you understand why we’ve made this difficult decision. The Firebug spirit lives on in all of the browser developer tools we build and use today.

The Firefox DevTools and Firebug teams