Thanks for all your help.
Unfortunately the suggested solutions are not good enough in my view.
As to the label method, I need to right-click | scroll and select | left-click. But in OE, it is just one click. Also it only highlight one post. In OE, it highlights all posts in the same thread. The advantage is when there's a new (unread) post added to the thread, I can see easily. It would be great if there's any shortcut key for it.
By the way, in OE, if I have highlighted any thread, it provides a notification function. When I select a newsgroup and download new posts, if there's new post added to any of my highlighted thread, the whole newsgroup will be highlighted.
As to the flag one, it is not as dintinct as the label. Also it loses all the function mentioned above.
It appears extensions for mail/newsreader are very few. For the extensions available for mail/newsreader, their existences are likely due to their browser counterparts - the authors develop the extensions for the browser, at the same time, they support mail/newsreader at their convenience.
After all, thanks so much for your help
UserAgent: Mozilla/5.0 (Windows; U; Windows NT 5.1; en-US; rv:1.7.8) Gecko/20050511 MultiZilla/1.7.9.0a