Using F (forward) and B (back) seems to work fine here with threads. Buttons can be added to the Mail Toolbar.

For me, F and B work the same as the Next/Previous buttons. They seem to work if I start in the middle of a thread but not on the first msg in the thread. I open the 1st msg, hit F, and it jumps past the rest of the thread.

I know buttons can be added, but there aren't any buttons that do what I want. (RealNextPrevious did exactly what I want.)

For my test, the folder was arranged with View/Sort by/Date & Descending & Threaded. N (next unread) and P (previous unread) also work properly in threaded view. You try Repair Folder from the folder context menu, Properties, or create a new profile and add an account in case there's something wrong with your current profile. Help/Troubleshooting, about:profiles.

I also arrange by date, descending, threaded. I want to read a mix of read and unread, so N and P wouldn't be appropriate. I tried Repair Folder with no change. Not a profile problem, I believe, since RealPrevNext worked fine until the Tbird update. (The update also made Priority Switcher add-on run very slowly. Makes no sense at all...)

How did you make RealPrevNext work in TB 60? It's only compatible with TB 3, although there are sometimes ways to make such add-ons work in newer versions. As for Priority Switcher, do you have the version compatible with TB 68?

Not sure how. There are 2 RPN versions, both incompatible with TB68. But one of them worked fine with the previous TB. Sorry, no screen shots because no earlier TB on my machine.

Priority Switcher is the TB 68 compatible version. Remember that TB doesn't allow use of add-ons that it deems incompatible.