Mozilla launched a Developer Edition of Firefox recently. The main idea was to create a version of Firefox that is optimized for web development tasks.
It is separate from other versions of Firefox and Mozilla switched the theme of that version to reflect that. It is not the only indicator though. You can run the Developer version side by side with your regular version of Firefox and both will run fine.
This did cause bit of a issue with existing Aurora users as users were moved to a new profile in the process which meant that access to bookmarks, the browsing history and modifications were not available after the update (read here how to fix that).
You can check out my first look at the Developer Edition of Firefox here.
Anyway, the Developer Edition ships with a dark theme enabled by default. Not only is it dark, it is also using squared tabs instead of round ones.
It is possible to switch to the regular theme in the Developer Edition as well if that is preferred.
If you like the dark theme and are using Firefox currently, you may find it interesting that it is possible to enable it in all editions of Firefox as well.
Update: Mozilla seems to have removed that option with the release of Firefox 40. This means that it is no longer possible to enable the dark theme in Firefox editions that are not the Developer edition. Interested users can download the theme as an add-on for the web browser though to add it to it. The information below are out of date and no longer valid.
Note: Enabling the theme on an existing profile may cause display issues due to previous modifications or old preferences. It is recommended to create a new profile for this. As you can see on the screenshot below, the title bar of the window is not painted black.
To enable the dark theme do the following:
You should notice immediately that tabs and other interface elements get painted dark to reflect the change.
To undo the change simply set the browser.devedition.theme.enabled preference to false again under about:config.
Add-ons may interfere with the theme as well. If you have installed an add-on that modifies UI elements, Classic Theme Restorer comes to mind, then you may notice display issues when switching to the dark theme. (via)
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