We have enabled the Multiprocess Browser Toolbox on all channels with Bug 1625937.
We should update the documentation to describe the specifics from the Multiprocess Browser Toolbox.

The mode switcher can also be explained and screenshots should be updated.

We can also use this occasion to document some advanced user scenarios, such as executing Chrome Privileged code in a content process, which is frequently asked about.

This most likely is not the place where I should ask, but not sure where else to ask it.

At some point in the last month or so, in the Browser Toolbox settings the option to 'Enable the Multiprocess Browser Toolbox' disappeared. Is there a way to restore that functionality via about:confg?

(In reply to Ed from comment #1)

Created attachment 9318493 [details]
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This most likely is not the place where I should ask, but not sure where else to ask it.

Best option is to either file a bug, or get in touch with us (eg discourse, mailing list or chat at https://chat.mozilla.org/#/room/#devtools:mozilla.org)

At some point in the last month or so, in the Browser Toolbox settings the option to 'Enable the Multiprocess Browser Toolbox' disappeared. Is there a way to restore that functionality via about:confg?

Short answer is no, we enabled the Multiprocess Browser Toolbox everywhere so this option is gone.
You should now have radio buttons on top of the Browser Toolbox window to select between parent process only or regular multiprocess mode.

OK, thanks.