You can no longer enable Firefox developer edition theme in the release channel from Firefox 40 onwards, unless Mozilla changes its mind. Till Firefox 40, Mozilla has allowed users on stable releases to enable Firefox developer edition theme in their browser by toggling ‘browser.devedition.theme.enabled’ preference in about: config.
That’s no longer the case now, Mozilla has switched how the theme is included in its lightweight theme manager, that preference now has no effect.
UPDATE August 15, 2015: This Add-on enables the Official Developer Theme in Firefox Release and Beta Channels
Reasons for not to include dev edition theme in release channels
If we included the Dev Edition theme as a built in lw theme for release, then it would have broken the use case of syncing two profiles between the two channels (Dev Edition and release).” Mozilla’s Brian Grinstead said in a bug filed.
” If lightweightThemes.selectedThemeID was synced to email@example.com on Release then it would unintentionally apply dev edition theme to people who didn’t want it. So in order to have a better technical solution (having the theme implemented as a lightweight theme), we had to give up this hidden feature for release channels.” he added.