Follow up to Bug 1495944, there is still a way for users to open DevTools on about:devtools-toolbox
The STRs have to be followed very accurately to reproduce it
- start firefox
- go to about:debugging
- debug/inspect a tab
- in the about:devtools-toolbox that was opened, select Tools > WebDeveloper
ER: The toolbox menu items should not be displayed
AR: The toolbox menu items are displayed (for instance Toggle Tools). Using them will open a nested devtools toolbox.
Note: if you open the Tools > WebDeveloper menu at any time before doing it in about:devtools-toolbox, the menu will be correct. It needs to be the first opening of the menu.