Howto: Stop Firefox Forcing Dark Mode on Websites
Here's how to stop Firefox automatically turning on dark-mode for websites.
In the address bar, type in about:config
and press ⏎ and accept the warning it gives you.
Add a new value ui.systemUsesDarkTheme
set it to type number
and pick one of the following:
0
to tell websites to always use the light theme.1
to tell websites to always use the dark theme.2
to tell websites you have no preference.

Dev Tools
If you want to do this on a one-off basis, open up the Dev Tools and click on the colour simulation icon to change your preferences.

It only works on a per-page basis and disappears when you refresh.
Why
I use dark mode on my laptop and most of my Linux apps. But I find most websites' dark mode to look a bit rubbish. By setting ui.systemUsesDarkTheme
to 0
, I get a nice looking light theme, which I can switch to dark if I want using the website's own controls.
I wish Firefox implemented this as an easy to use toggle switch - but this method works if you're happy to fiddle around in Firefox's guts.
about:config, Firefox’s hidden gem.
@Edent i get migraines. And I hate dark mode.The irony here is that its not about light, its about contrast with the surrounding environment. Light mode is less hurtful to migraines during the day.