Force Firefox to light mode
This is mainly for my own future reference. You can configure Firefox to use a light theme, but that doesn't actually mean the configuration pages are shown as dark text on a light background.
Why? Probably because the color settings of the operating system overrule a manual setting in the application. Looks like a bug to me.
To fix this a setting is required. Open about:config (more info about that) and find the setting ui.systemUsesDarkTheme. Change the value to 0 (zero).
Now this setting is so advanced it may not even exist in about:config, like on my installation. In that case create it with a the type Number. Set the value to 0.
Value 0 forces Firefox to always use Light theme mode.
Value 1 forces Dark theme mode.