In Firefox press Shift+F2, then type "screenshot --fullpage" in the command line at the bottom of the page. No addon, plugin or extension needed.

I'm not sure how jd3 did it but I use a Chrome extension. If you just google full page screenshot Chrome you will see several options. There are also choices for Firefox as well. Here is the one I use on Chrome:

https://chrome.google.com/webstore/detail/full-page-screen-c...

hello! I used screengrab on SeaMonkey ;)

https://addons.mozilla.org/en-uS/seamonkey/addon/screengrab-...