On my system, at least, a message can be starred or unstarred without affecting the read status, if the Message Pane is hidden (F8). If the Message Pane is open, the same result applies as long as the star is added or removed from the message right-click menu (Mark/Add Star).

Thank you for the insight. I tried what you described and it did not resolve my issue. Whether I right click to add the star or click the star, it results in the same issue. The only time I can add a star and not have it result in the message going back to unread is if I open the message and leave it open for a significant amount of time, then if I star it, it does not mark it as unopened.

Don't know if this matters, but under Tools/Options(Mac:Preferences)/Advanced/Reading & Display, I have "Automatically mark messages as read" and "Immediately on display" checked.

I have both of those checked as well.

You could try repairing the folder (right-click, Properties, Repair Folder) and see if the correct read status is maintained. Be sure to back up your mail or the entire profile before you do this, as in rare cases the repairing results in vanishing mail (which points to the folder being corrupted in the first place).

None of this has worked. Thank you for your suggestions though.

If the folder is corrupted beyond the simple repair operation, I suggest rebuilding it according to these instructions:

http://kb.mozillazine.org/Compacting_folders#Real_fix

Essentially, you move the messages out of the folder and replace it with a new version.