The current Nightly versions of Firefox 35 (planned release date: January 6, 2015) features a new theme interface within the Customize mode (Firefox Menu > Customize or right-click on a toolbar and select Customize…). Added to the Customize mode is a new Themes button with a pop-up menu giving you some options for managing and installing themes:
- You can switch between themes you have already installed (as seen in Appearance section of the Add-ons Manager).
- Choose to install and switch to one of the recommended themes.
- Go to addons.mozilla.org Theme Library to browse and download/install a new theme.
When I first saw this, I really didn’t understand the purpose or need of this change. But after playing with this feature a little bit on my Nightly Build profile it started to make more sense. Switching your theme (or installing new ones) via the Customize mode (you are Customizing Firefox after all) is more logical than going into the Appearance section of the Add-ons manager. As of right now, this feature is still in development and you can still access your themes the old way via Tools > Add-ons > Appearance.