Some feedback:

While I like the related thread and related song dropdowns, I feel the Quickjump dropdown, replacing the explicit message list, is less useful and less efficient. With the explicit list, one could often tell at a glance (with shorter threads) who added the last message and when. Even as the message list grew, it was easier to skip to the end of the listing with the Page Down button or mouse scroll wheel, without having to first open a dropdown (whose visible area is also more limited).

The explicit listing also allows us to copy a link to a particular message, for adding cross-reference links in new posts. The dropdown leaves no reasonably straightforward way to achieve this. See the other recent thread on updating Mudcat for my further comments on this subject.

The dropdown does conserve some space, so there is less stuff to skip past, but since you usually have to scroll anyway (or reload the page with the "Sort Descending" link) it's still more efficient not to have to use a dropdown. Often, checking the last entries was sufficient to know whether to bother scrolling or jumping down to the newest messages, and in the case of scrolling, was a consistent continuation of the operation you were just doing.

For the quickest quickjumping, you'd list the dropdown entries in the reverse order of the messages, so the first entries jump the farthest.

If the Quickjump dropdown is retained, the label should be expanded, so it's clear what one is quickjumping to: "Quickjump to thread message"? I would also list the date first, to make them easier to scan; the user names would still list relatively straight down.