Setting up an extension development environment - Archive of obsolete content
devtools.chrome.enabled = true.
...the devtools.chrome.enabled preference must also be set to true for the browser debugger to be enabled.

Debugging JavaScript
to enable it, go to about:config in the url bar and set devtools.chrome.enabled to true, or set the "enable chrome and add-on debugging" option in the developer tool settings.
... go to about:config and set the following two prefs: devtools.chrome.enabled: true devtools.debugger.remote-enabled: true after you restart the browser, you can access the browser debugger through tools > web developer > browser toolbox.

Browser Console - Firefox Developer Tools
to enable it set the devtools.chrome.enabled preference to true in about:config, or set the "enable browser chrome and add-on debugging toolboxes" (firefox 40 and later) option in the developer tool settings.
...to enable it set the devtools.chrome.enabled preference to true in about:config, or set the "enable chrome debugging" option in the developer tool settings.

Tutorial: Show Allocations Per Call Path - Firefox Developer Tools
visit the url about:config, and set the devtools.chrome.enabled preference to true: setting the 'devtools.chrome.enabled' preference open a developer scratchpad (menu button > developer > scratchpad), and select "browser" from the "environment" menu.

about:debugging (before Firefox 68) - Firefox Developer Tools
the "enable add-on debugging" button works by turning on the devtools.chrome.enabled and devtools.debugger.remote-enabled preferences.