privacy.resistFingerprinting;true enables/disables many features to prevent fingerprinting so it does break a lot of things and I wouldn’t be too surprised.
I wasn’t aware of dark mode breakage (which would set you apart from default and thus be an item for fingerprinting), but I don’t use it due to webapp shortcut breakage (in TheLounge) and text-to-speech disabling (Clozemaster, audio captchas potentially).
If i’m right, this addon plays in the CSS (client side) so again i’m not sure (someone correct me) but the server does not know you changed the color in first place (but maybe they got other ways to track addon tho)
This comment supposed to make you feel more good about using the addon but i think i made you more scared
As written in the Mozilla documentation, setting privacy.resistFingerprinting to True results in Firefox always loading the pages with their “light” theme. Optionally, you can go to the about:config in your Firefox and overwrite this. For instance, instead of always loading the “light” theme, Firefox would always load the “dark” theme. Keep in mind that a website must define a dark theme; otherwise, nothing will happen.
The Reporting API (the current Content Security Policy and other headers will use this in the future) is primarily built for web developers to improve their custom web applications, not for tracking parties.
To detect this potential behavior, you can do at least:
If you have set privacy.resistFingerprinting to true, prefers-color-scheme preference is overridden to light. Alternately, users can create the numeric preference ui.systemUsesDarkTheme to override the default behavior and return light (value: 0), dark (value: 1), or no-preference (value: 2). (Any other value causes Firefox to return light.)
→ ui.systemUsesDarkTheme: 1
Exactly, the Media Queries Level 5 specifies this feature, but it is still a draft. According to the MDN page, Firefox for Android and Internet Explorer don’t support this CSS feature.