Firefox, dark-mode, and fingerprinting

Thu 30 Sep 2021

Here's a little thing I noticed. As the season turns, I've switched my working environment from light mode to dark mode. But I noticed that a lot of websites that I *know* have dark modes aren't picking it up, and I wondered why. My original thoughts were that it had something to do with the fact that GTK/GNOME don't technically have a system dark mode yet; they just have dark themes. But Firefox *was* picking up that it was time to use its dark UI theme, it's just websites weren't seeing it.

So, here's the answer: since a year or two, Firefox's privacy settings have included a "Resist fingerprinting" feature. It's available in preferences under Privacy, custom settings. I had it on; it breaks a number of "web platform standards", but sod them, right? Anyway, it turns out that Firefox disregards the CSS prefers-color-scheme media selector when resist fingerprinting is turned on.

There was a suggestion to enable some kind of override, but it was overruled. If web site dark modes are important to you, you have to go without fingerprinting mitigation.