Firefox for iOS offers an "Enable Night Mode" toggle which not only darkens the Firefox interface, but websites as well.
While not available for desktop OSes yet, you can achieve a similar result with ShadowFox (macOS users can install via Homebrew or MacPorts) and Dark Reader. ShadowFox correctly darkens every UI element, including ones that other themes can have trouble with, like the address bar, context menu, and history page.
/misc | Oct 23, 2019