Comment by azernik

Scroll down a bit farther to see the full comment tree.

"Next" goes approximately down the tree in the order it's displayed on the page, by depth-first search.

"Prev" just reverses the same process as "Next".

"Parent" differs from "prev" in that it goes to the parent e-mail even if this email has earlier siblings.

(Generally, I just scroll down to the tree view and click around manually.)