You can try to create a new Boolean pref on the about:config page and set its value to true.

• ui.systemUsesDarkTheme = true

• https://support.mozilla.org/en-US/kb/about-config-editor-firefox

• https://developer.mozilla.org/en-US/docs/Web/CSS/@media/prefers-color-scheme

See also:

• Bug 1494034 - Add support for CSS prefers-color-scheme media feature
• Bug 1529323 - Make prefers-color-scheme respect the Firefox theme rather than the system theme

(please do not comment in bug reports
https://bugzilla.mozilla.org/page.cgi?id=etiquette.html)

I can likely make a temporary fix for this myself with your suggestion, but none the less this is definitely a bug in Firefox. Other major browsers correctly reflect the system darkmode.

(and, moreover, as a web developer, I have no control over the configs a user in Firefox has set, so this will still make my website behave incorrectly)

Hm, now that I think about it... this is actually maybe a bug in the browser theme.

https://bugzilla.mozilla.org/show_bug.cgi?id=1529323#c9

I have my theme set to "Default," but the browser is, I just noticed, still in dark mode. So the browser is not correctly reading the system theme, and the website is responding to the browser theme.

I'm not able to upload video here, but I've just confirmed that if I switch macOS to light mode, firefox switches to light mode; if I switch it to dark mode, firefox switches to dark mode; but if I switch it to auto, firefox does not respond to the change, and remains in either dark or light mode, which ever was last set.

I had my os in dark mode on Mojave, so when I switched to auto on Catalina, dark mode was the last one set. This means Firefox has been in dark mode ever since.

So the bug here is that Firefox doesn't know what to do when a user's mac is in "auto" darkmode.