If using another browser on a PC, you may have to take screenshots the old fashioned way, one section at a time as you scroll/page down.  To do this, hold down Alt-PrintScreen.  Upon releasing the keys, a screenshot will be taken and automatically saved to your documents folder.  Once saved, you can open the image in your drawing/painting application of choice and mark it up.

For more information/diagrams of how to take screenshots on a PC, refer to http://wikihow.com/Take-a-Screenshot-in-Microsoft-Windows