If using another browser on a Mac, 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 Shift-Command-4.  Upon releasing the keys, tap the spacebar to indicate you want to capture an entire visible window; then move your mouse over it until it turns to a shade of gray, and left click once.  This will automatically take a picture of the selected window and save it to your desktop.  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 Mac, refer to http://wikihow.com/Take-a-Screenshot-in-Mac-OS-X