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Re:Firefox Preview/GeckoView Add-Ons Support by jumbopapa on Thursday October 24, 2019 @08:55AM (#21344166)
This is likely Twitter adhering to browser dark theme. OP wants Firefox to handle the dark theme itself and convert all web pages.
Re:Firefox Preview/GeckoView Add-Ons Support by cpeterso on Thursday October 24, 2019 @03:09PM (#21348100)
Here is the GitHub issue requesting Firefox Preview force dark mode. Firefox for iOS has some CSS hacks to support this, but it can break some websites.

https://github.com/mozilla-mobile/fenix/issues/2139