sepsis-inspector is an interactive inspector for debugging Mozilla toolkit applications, extensions and web pages. It's part of the sepsis project.
AMO reviews can be slow, so make sure to check for newer versions listed on the page showing the full version history. The most current version is sepsis-inspector 0.5.
Read the sepsis-inspector owner's manual (somewhat not up-to-date; written for version 0.3).
Get the code:
sepsis-inspector issue tracker on GitLab