You can’t turn that beautiful dark theme developer edition offers on in Firefox beta and release channels from version 40 onwards, Mozilla has removed dev theme support and the preference ‘browser.devedition.theme.enabled also, this applies to Nightly version too.
Luckily Developer Edition theme enabler add-on activates the much wanted theme in Beta and stable versions, but not in Nightly.
Enable Firefox developer edition’s dark theme in Nightly
1. Visit about:config
2. Find the preference devtools.theme (double click) and change its value to dark from light.
3. Select Add-ons from the menu, Appearance, click ‘Enable‘ button for ‘developer theme’
You can easily switch between the light and dark themes from Appearance section in Add-ons manager.