This is an issue i struggle with every time, i have tried a number of extensions but the capture does not always come out the way i want. Today, i googled again and found this link that helped and i was satisfied with the outcome.

https://www.softwarehow.com/take-full-webpage-screenshot/

Please note, i went with the Snagit option, and a little update; to do this in 2018 snagit, click on capture, under image; selection; select panoramic, if you don’t find it there, look to the preset options below the opened widget and add panoramic scrolling, afterwards you did get a few seconds video that will show you how to use it.

(Warning – Snagit is not free, it comes with a 15 days trial however)