TL;DR What does FF sends over the network upon start up and why it's not as bad as Twitter/any social networks want us to believe.

Press MAJ + Fn + F2 to open the firefox console then

screenshot --fullpage

Amazing!

How to capture a screenshot of a full webpage (even with scrolling):

1) Open the firefox command line with SHIFT + F2
2) Type in the command: screenshot --fullpage <filename>
3) Enjoy!