(Spawned from Bug 1521486)
When inspecting local addons, we have to use a separate Firefox instance, running with a separate profile, because addons are running in the main process and to debug this main process, we should be in another instance of Firefox.
To do that we reused the toolbox-process-window, but this was originally intended for Firefox UI developers. It is normally used for the Browser Toolbox. And it creates a default profile with a set of preferences that can be confusing to addon developers: https://searchfox.org/mozilla-central/rev/6c784c93cfbd5119ed07773a170b59fbce1377ea/devtools/client/framework/toolbox-process-window.js#103-114
We should use another profile when we want to debug addons, that doesn't show unnecessary Firefox internals to the developer.