After updating Firefox to version 50.0 my Firebug opens the default developer tools. The original Firebug doesn't work anymore.
I have always preferred Firebug as my default debugging tool. I want the original Firebug back in Firefox 50. How can I do that?
Firebug does not work anymore once multi-process Firefox (separate processes for the Firefox UI and the websites) is enabled. See the related post in the Firebug blog.
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. Though at some point this preference will be removed.
So, you are advised to use the Firefox DevTools instead. I am writing a migration guide on MDN. Firebug features that are not in the DevTools are covered in a Firefox bug.