about:blank in order to maintain a universal dark theme. An unfortunate consequence of this is that empty iframes on webpages are unintentionally styled (see here). No Empty iframes is a simple extension available on AMO that solves this problem by adding a content-less
div to the body of any empty
Below is a simple test for No Empty iframes. The first iframe has no content, and the second does. Both iframes should have a green background, but with just ShadowFox installed, you will notice the first is grey.
With No Empty iframes installed (or ShadowFox uninstalled), both boxes should be green.
No Empty iframes is very simple extension, and the following code could be easily be applied via an extension like Violentmonkey.