How to change screen resolution in Firefox browser?

Question

Screen resolution

I have the Vertical Tabs extension in my Firefox browser - a very handy thing for modern widescreen monitors, especially when at least a hundred tabs are constantly open at the same time. However, there is a problem, in that most sites do not want to know anything about the fact that screen size and browser window size are not the same thing at all. As a result, many so-called "responsive" sites do not display correctly. The question is what parameter webmasters usually use when writing website engines to determine the screen width and how can this parameter be changed?