I can print a webpage to PDF, but there are times when I need to take a screenshot of the entire length of a webpage. For example, a lot of formatting is lost when printing. So, a screenshot is better since it gives you the entire length of the webpage exactly how it is rendered in the browser. Thankfully, there is no need to install any extension to do this in Firefox.
Enable the screenshot option: Press
F12 to open the Developer Tools. Click on the ⚙ (gear) icon on the right. In the Available Toolbox Buttons section, enable the Take a screenshot of the entire page option. A 📷 (camera) icon will appear to the left of the ⚙ (gear) icon. From now on, this 📷 (camera) icon will be available in Developer Tools.
Take a screenshot: Open the webpage you want to take a screenshot of. Press
F12 to open Developer Tools and click the 📷 (camera) icon to take a screenshot. It will be saved in your Downloads folder as a PNG image by default.
Tried with: Firefox 53.0.2 and Ubuntu 16.04