I always wondered how people do this without an addon or plugin, and I finally know how! I’ll never remember it though, so here’s a blogpost on it.

  1. Press “SHIFT” and  “F2” buttons at the same time to bring up the developer console.
    Open the developer's console.

  2. Type “screenshot –fullpage” into the box or “developer console”.
    Enter the commands.

  3. Press enter! It saves to your default Downloads folder. You can see the file path at the top.
    Press enter!

Now you can upload entire screenshots from 4chan or Reddit or whatever odd conversation you want to capture!