Apologies if there's already a way to do this; I looked and couldn't find one, but I might have overlooked one.
The "jump to first unread" button and similar features appear to assume that you have read all posts prior to the latest post you've read. For example, if a thread is 80 pages long, and you have clicked on pages 1, 2, and 78, using the button will typically take you to page 79. This makes it easy to lose your place while backreading, as well as posing a problem if you leave midway through reading a page (as all posts on that page will register as read).
My suggestion is to add a button that would mark a particular post as the "last post read" for the purpose of these features.