This page is dedicated to Firebug 2.0 release. Read more to know why you should install it (currently in alpha phase).
Download Firebug 2 alpha
Firebug 2.0 release introduces many improvements where the most important one is probably support for new debugging engine in Firefox known as JSD2.
See the following compatibility table between Firefox and Firebug:
Firebug 1.12 is based on JSD1 (the old debugging engine) that is going to be removed in Firefox 31. This version will continue working fine except of the Script panel and debugging features. Other panels will work just fine. If you are JS developer and you want the Script panel to debug JS install Firebug 2 alpha.