I’ve used this in the past - if I remember correctly, it outputs HTML, but you could probably take a scrolling screenshot of that afterwards
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I have never used discord so my idea might be worthless. If there's a way to display whole conversation on one page you can make the whole page screenshot - https://support.mozilla.org/en-US/kb/take-screenshots-firefox?redirectslug=firefox-screenshots&redirectlocale=en-US#w_what-types-of-screenshots-can-i-capture
unfortunately, discord won't let me scroll, thus effectively making this non-viable
edit: whole page screenshot won't work either. I just tried, it would just capture the visible tab and quit