I don't think there is working replacement...
My comments/questions:
- What's your use case, are you using Selenium?
- It should be an extension to DevTools directly (there is no need to depend on Firebug unless you want support the Firebug theme)
- My gut feeling is that having an extra new panel for such small feature is too much. It would be better to have simpler UI if possible.
Let me know if you need an example extension that shows specific part you need to do.
Here is a list of existing examples:
Honza