As the Omniscient Browser Toolbox progress, the Browser Content Toolbox becomes useless.
This toolbox is the one you open from "Web Developer" Menu and "Browser Content Toolbox" entry.
It opens a special toolbox, with just the console, debugger and memory, which are debugging the content process into which the current tab is running.

The Omniscient Browser Toolbox allows to debug the parent process and all the content processes, all at the same time, in a unique Toolbox. This makes the Browser Content Toolbox irrelevant and unecessary limited.

Before removing the Browser Content Toolbox, we probably should:

• enable the JS Context selector by default in the Browser Toolbox (bug 1628346), so that we can still evaluate JS in the content processes,
• enable the Multiprocess Browser Toolbox on all channels (bug 1625937), so that everyone can debug all processes, on all channels.

(In reply to Alexandre Poirot [:ochameau] from comment #2)

Before removing the Browser Content Toolbox, we probably should:

• enable the JS Context selector by default in the Browser Toolbox (bug 1628346), so that we can still evaluate JS in the content processes,
• enable the Multiprocess Browser Toolbox on all channels (bug 1625937), so that everyone can debug all processes, on all channels.

Bug #1 has been fixed already.

The Browser Toolbox should be handy enough now with parent versus multiprocess modes
to debug the content process. The context selector in the right of the console evaluation
of the browser console and toolbox allow to select the content processes.
The thread selector in the debugger also allow to distinguish content processes.

https://hg.mozilla.org/mozilla-central/rev/f39720747d46