Perhaps I am not understanding you correctly. Firefox Web Developer Tools shows css breakpoints [0] much the same as getcssscan.com [1] - here the examples are with the h3 logo class. Firefox WDT actually shows the entire css cascade.
And back to the original question, how do you copy the font size from the breakpoint rules, the margin from the second set of rules, and the font family from the third, with a callout to the breakpoint, in a single click using Firefox dev tools?