This doesn't sound smart, it is contrary to how Dark Mode works everywhere else across all other apps.
A hidden about:config option for this is fine, but it should not be default.
Users of the the Firefox Developer Edition for example, just because it has a dark theme as default, would get a really different web experience than users of the other versions.
Normal users would try some of the other themes and Dark Mode support in FF would apparently break. They would enable Dark Mode or it would be auto-enabled (based on things like time or sunset), but Firefox would stay in light mode, giving away the impression of an outdated application without dark mode support.
I don't really see the need to change the current default.
(In reply to Andrew Kelley from comment #19)
I agree this is how it should work when Firefox's "default" theme is chosen, and that this should be the default for Firefox. However note that detection of the system theme is not always available or correct. On my system - XFCE with a dark theme - Firefox incorrectly determines that this is a light theme. [..] As a user it was completely surprising that I could have "dark" theme selected while prefers-color-scheme reported "light".
A better way to solve this, apart from just improving the detection methods, would be to give the users to override what color theme (dark/light) their themes are.