A folder in Folder Pane will display a new mail star if new mail has been received since you last opened that folder. But after opening that folder the new mail icon will be removed from the folder.

If you select a folder with new mail, then the individual emails that are new will have a green unread marker and a yellow star. Example: So you selected the folder with new mail -and can see two with unread and star. If you choose not to read them and select another folder and then return back to the folder to read them then you will find they are no longer marked with a yellow star because they are no longer considered new mail since you last selected the folder, they are just unread mail.

So the new star icon on email will only be available temporarilly.

This answer helps me understand what thunderbird is doing with my new email and the yellow stars, but I would like the yellow stars to remain for at least 24 hours to show me what I, myself, consider to be new emails. There should be an option for that. Perhaps I can find out where the thunderbird setting is for that and create an extension to do what I want thunderbird to do.

Your definition and Thunderbird's are obviously different and there is no way to configure it as "New" is defined in the IMAP standard and Thunderbird follows that standard, even for POP accounts which do not intrinsically have a "new" status.

You could however use a saved search folder to do what you want. This link is probably 10 years old, but the information is still fresh, even if the images are not as the underlying search has not changed in that time. (use the crtrl+Shift+F keystrokes to open the search dialog). http://www.freeemailtutorials.com/mozillaThunderbird/searchingForEmails.php