On Firefox, taking a screenshot is just a shortcut away [Wayback/Archive] Take screenshots in Firefox | Firefox Help:
Ctrl+Shift+Son Windows/Linux/Chrome OSCommand+Shift+Son MacOS
Too bad Mozilla also appears to be amongst the bad guys in firing an employee having cancer ¹.
On Chrome not so much and require opening the Chrome Developer Tools which take up precious screen estate, and saves the screenshot in the downloads folder instead of the clipboard. Many guides exist, for instance:





