I meant this to be a question because I was having a hard time figuring this out. But I eventually found something that worked for me. I want to share this because the problem is both annoying and bizarre, and I couldn’t find another answer online.
What worked for me is changing the text area’s “height” value using the Firefox inspector. This is the tricky part. You have to identify the container element of the scrollable content which should have a fixed height, and tune it to a higher value (high enough to contain the whole content, like
10000px). Now “Print as PDF” will save everything to the final PDF. Firefox’s “Take a screenshot” will also register all the content, but there seems to be a catch. It has two buttons, “Download” for saving the screenshot as a file and “Copy” for saving the picture to the clipboard. The catch is that “Dowload” doesn’t work, at least for me. The final image is just a black rectangle. On the other hand “Copy” works: open GIMP,