If you are reading the contents of a newsgroup, and try to place a "watch thread" on an item that has no followups, either by pressing "W" or using the menu "Message | Watch Thread", nothing happens -- the request is ignored. This function works fine for threads that have more than one followup.
The version is Mozilla 1.1 alpha.
It seems to work fine with the "Modern" skin. I was using the Orbitz skin before. I'm assuming that the Orbitz skin didn't have an icon to represent a watch on a one-item thread, and thus appeared to ignore an "add watch" under that circumstance. Maybe nonexistent icons in skins should at least be substituted with icons from one of the default skins.
*** This bug has been marked as a duplicate of 122640 ***
marking verified as a duplicate