The best I can think of is to printscreen with other threads closed and that specific one expanded.
Depends on how much is going on in that page.

(printscreen->save to pdf, specifically)

Bluh okay, I was afraid of fhay

*that

Hmm - can you click on the comment's HTML link, for where the thread starts?
(though if the problem is not being able to access the thread in the first place... yeeeeah, you're SOL there. :< That sucks.)

I can access the whole thread, it's just really long lmao

Ahhhh.
It may take multiple screencaps, then.

If there's a link to the entire thread, I'd try right-clicking on it and saving the page