You want Selenium
Selenium automates browsers. That's it! What you do with that power is entirely up to you. Primarily, it is for automating web applications for testing purposes, but is certainly not limited to just that. Boring web-based administration tasks can (and should!) be automated as well.
Selenium has the support of some of the largest ...
Firefox has now a built-in "Take a screenshot" tool in address bar (Click the three dots). It has full page screenshot and visible page screenshot. Can copy to clipboard or download. Pretty convinient!