As we’ve said before you can no longer activate the Firefox developer edition’s beautiful dark theme in current Firefox 40 stable channel or future release versions by visiting about:config and toggling the ‘browser.devedition.theme.enabled’ preference value.
Because Mozilla has removed the support for the theme (we doubt they may not add it back). If you’re about to switch to dev. version for that, wait! A Mozilla contributor has created an add-on ‘Developer Edition theme enabler’ and is available on AMO, this enables the dev. theme in the release and beta versions .
Developer Edition Theme Enabler Add-on for Firefox
Firefox Release version: Install the add-on, visit ‘Appearance’ in about:addons page and click ‘Enable’ button for ‘Developer Edition (disabled)’.
Don’t worry about the half-visibility of that close (x) button on the active tab, however, it appears fully when you hover over it, it’s a bug in Firefox 40 (not add-on’s fault), will be fixed in version 41.
For beta version : after the add-on install, the dark theme will be automatically activated, but you need to restart the browser for it to visible.
What changes the add-ons makes?
This add-on adds an entry ‘Developer Edition’ in the Appearance and sets ‘devtools.theme’ about:config preference value to ‘dark’.
Removing the developer theme
Uninstalling the add-on removes the dev theme and enables the default. This add-on worked perfectly for us without any issues on Beta and Stable channels.