Add-ons can implement new developer tools that integrate into the normal devtools toolbox. However, AFAICT there's no way to get these loaded into the Browser Toolbox. Which is a shame, because that's exactly what I want to do.

The API does not treat addons differently. If an addon has registered a tool, based on the output of isTargetSupported method, the tool will be shown or hidden from various different target types. See the currently registered default tools for some idea [0] . See line 339, which is for the Storage Inspector which hides itself from Browser Toolbox only allowing local tabs. How is your add-on registering the developer tool ? [0] http://dxr.mozilla.org/mozilla-central/source/browser/devtools/main.js

Well, yes, this is true. But since this is an add-on and the Browser Toolbox has it's own separate profile, the add-on is never loaded, right?

Well, true. Unless the addon is added manually to that profile.

Maybe we could copy the existing add-ons over to the Browser Toolbox profile on creation, like we do for prefs.js: http://dxr.mozilla.org/mozilla-central/source/browser/devtools/framework/ToolboxProcess.jsm#154