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.
Press “SHIFT” and “F2” buttons at the same time to bring up the developer console.
Type “screenshot –fullpage” into the box or “developer console”.
Press enter! It saves to your default Downloads folder. You can see the file path at the top.
Now you can upload entire screenshots from 4chan or Reddit or whatever odd conversation you want to capture!