How to get text/body content when in Reader mode? (firefox)

[philipsd6]

With Firefox, it's possible to toggle reader mode for a tab, but I don't know how to get the content of a tab in reader mode:

wc -l mnt/tabs/last-focused/text.txt
288 mnt/tabs/last-focused/text.txt
echo toggleReaderMode > mnt/tabs/last-focused/control
wc -l mnt/tabs/last-focused/text.txt
wc: mnt/tabs/last-focused/text.txt: Input/output error

Putting a tab into reader mode, changes the url to about:reader?url=https://original/url. The Firefox developer tools shows the HTML content, and in the console, I can display document.body.innerText just fine, but it doesn't work in TabFS.

Anyone have any idea how to do this?