Question: I've opened a Firefox today, hit the Firebug button and it showed me that in the new version it is integrated into the developer tools.

I hit ok and realized that it doesn't work at all (everytime using *Inspect Element* the `<body>` element is selected). So I would like to revert that setup back, but I cannot find where to switch it. I tried to reinstall Firebug, but that setup is stored somewhere so it automatically starts the DevTools instead of Firebug.

Any help how to revert back to the classic Firebug?

Answer: 
[As I wrote in the Firebug discussion group](https://groups.google.com/d/msg/firebug/PiCXxzDdsO8/7usaqu6sDwAJ), that the `<body>` element is selected first before the actual element you inspected is a bug and I've already [filed a bug report](https://bugzilla.mozilla.org/show_bug.cgi?id=1324254) for it.

There is also a [bug related to the *Inspect Element with Firebug* option](https://github.com/firebug/firebug/issues/8077), which opens the DevTools, but doesn't actually select the inspected element. This bug got fixed in Firebug 2.0.19 by removing the option (so there's just the *Inspect Element* option left, which works as explained above).

As mentioned in [my answer to a related thread](https://stackoverflow.com/a/40748127/432681), you may be able to reenable Firebug by setting the preferences `browser.tabs.remote.autostart`, `browser.tabs.remote.autostart.1` and `browser.tabs.remote.autostart.2` to false via `about:config`.

These settings control whether Firefox works in [multi-process mode](https://developer.mozilla.org/en-US/Firefox/Multiprocess_Firefox) (separate processes for its UI and the websites). Unfortunately Firebug is incompatible with that mode.

**Note:** This is a temporary solution, because the multi-process mode will be mandatory at some point and the preference will be removed. Also, starting from [Firefox 57 only WebExtensions](https://blog.mozilla.org/addons/2016/11/23/add-ons-in-2017/) will be supported and Firebug is based on different APIs, so it definitely won't work anymore at that point.

Another way is to [install an old version of Firefox](https://ftp.mozilla.org/pub/firefox/releases/) like 48, [disable automatic updates](https://support.mozilla.org/en-US/questions/753852) and install Firebug there. Though, having said that, this option is not recommendable (at least not long term), because you will miss important (security) bug fixes and new features.
In additional to @Sebastian Zartner's answer, (*from [Firebug](http://getfirebug.com/)'s website*):

> 
> Firebug 2 doesn’t work in multi-process browsers (i.e. e10s) and converting it is too complex, so it will stop working when e10s is activated in Firefox. **You will be able to disable e10s for a while to keep using Firebug 2, but this won’t last forever**.
> 
> 
> 

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More about your question:

* [Firebug is not working with Firefox version 50.0](https://stackoverflow.com/questions/40800173/firebug-is-not-working-with-firefox-version-50-0)
* [Why got Firebug removed after updating Firefox to version 50?](https://stackoverflow.com/questions/40744448/why-got-firebug-removed-after-updating-firefox-to-version-50)