Behavour when pressing space on last message in thread.

When pressing spacebar Thunderbird always moves on to the next unread message in the folder. This happens whether it is in the same thread or not. I previously used Turnpike; in that software if you were in the last unread message in the thread and pressed space it would return to the thread view for the newsgroup or mailing list you were reading, the next unread keyboard shortcut operated only within the thread. I would like to set Thunderbird to behave like this. I cannot find any end of thread behaviour setting to stop Thunderbird from opening the next unread message if it is in a different thread. There is such a setting for folders (mailnews.nav_crosses_folders) something similar for threads would be extremely welcome as I find the default behaviour puts me off using Thunderbird.