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.
Original URL: http://feedproxy.google.com/~r/feedsapi/BwPx/~3/7GP0T0GrIII/