Here's the Twitter homepage in Firefox Preview with dark (device default) vs. light theme: https://i.imgur.com/T2KknGa.png

What else are you asking for?

This is likely Twitter adhering to browser dark theme. OP wants Firefox to handle the dark theme itself and convert all web pages.

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

Websites are complex, you cannot make a script that edits every page to make it both dark and readable when things like the background could be light colored images and fonts that are permanently dark exist

It's actually feasible for most websites, with surprisingly good results. CSS tricks go a long way, even if the website doesn't support prefers-color-scheme.

Give this addon a try (on desktop or mobile Fennec only, obviously): Dark Background and Light Text [0]

[0] https://addons.mozilla.org/en-US/firefox/addon/dark-backgrou...

https://darkreader.org/ works really well, in my experience.