per slack chat with jdescottes

BrowserToolbox has a separate profile from the browser, so preferences managed from about:config have no effect on BrowserToolbox. For example, if I flip devtools.browsertoolbox.fission to true in about:console, then omniscient browser toolbox is not turned on, which is confusing, since about:config is often considered as a single point to manage browser preferences.

A separate profile for BrowserToolbox is a technical restriction, caused by the facts that BrowserToolbox is running as a separate Firefox instance (to make debugging possible), and two Firefox instances can't share same profile.

There are few options to make BrowserToolbox preferencing less confusing:

• Remove BrowserToolbox specific preferences from defaults in Firefox, and make sure the documentation clearly describes how the user should change preferences. This is the easiest solution and obviously less confusing, but still it doesn't look clear for those who relies on about:config as a single place to manage the browser.
• Copy BrowserToolbox preferences from Firefox profile to BrowserToolbox profile, when Firefox starts. This option requires to restart the browser, which is sad, but might be better than #1.
• Keep the browser preferences and BrowserToolbox preferences in sync: whenever the user changes a pref in about:config it is changed in BrowserToolbox. It'd be the option of my choice from usability perspective, but might require some work.

Note that in Bug 1603361 we implemented the synchronization mechanism for devtools.browsertoolbox.fission which was the initial reason why we discussed about this.