The current Firebug version in development is 3.0 (aka Firebug.next).
 Download Sites
Next generation of Firebug is built on top of the Firefox native developer tools and the Add-on SDK.
- Use an existing infrastructure of native developer tools to solve issues related to e10s and remote debugging.
- Closely cooperate with Firefox DevTools team (and other teams like JetPack and extension developers).
- Use Mozilla infrastructure for running automated tests.
- Customize theme theme/layout of existing native tools and implement advanced features.
See Firebug.next wiki on github.com for detailed information.