Are you sure your not using Private Mode as that will have black background. And since you don't show screenshot of what your doing anyone reading will not know what to look for or look at?

I attached 5 screenshots with dialogs where 4 with dark main window and 1 not dark as it ought to be. So, how completely disable main window darkening(shading)?

The darkened pages are browser pages. The other page is a web page (on this site). First, I suggest you check your theme on Add-ons and Themes (ctl+shft+a). You may need to switch to a light theme. You can also do that on the customise page (right-click the 3 bar menu or toolbar and select Customize Toolbar.

If that doesn't fix it, try these troubleshooting steps.

https://support.mozilla.org/en-US/kb/diagnose-firefox-issues-using-troubleshoot-mode https://support.mozilla.org/en-US/kb/troubleshoot-extensions-themes-to-fix-problems (also deals with hardware acceleration)

Darkening is a security measure, especially if the user interface is included, to prevent websites from spoofing such a window to trick you. You can possibly hide such an overlay and restore the opacity via CSS code in userChrome.css, but this is not advised.

• https://www.reddit.com/r/FirefoxCSS/

cor-el, can you say what I need to write to userChrome.css?

Not important for what darkening added. This idiocy must be removed everywhere