when I open a thread, the new threads and threads with new posts are of course yellow
You mean "when I open a forum" there, right?
Open a forum and spot a yellow folder. Now click refresh. The folder should change back to not yellow without your ever having opened it.
I can't reproduce that in Safari or Camino or Firefox. When viewing a forum, the yellow folder (and red numbers) indicating unread posts don't change after clicking Reload.
I have about nine threads here that I read daily. I usually go into safari and do an "open in tabs" to open all nine of them at once.
As above, I take it you mean forums there, not threads.
So I just closed the window and reopened in tabs again. ALL forums showed no unread threads now. (simply having listed the forum threads has cleared the unread flag on all threads in each forum)
I can't reproduce this either, in Safari or Camino (haven't tested it in Firefox). If I quit and relaunch Safari or Camino though, instead of just closing the window, then I get the same unread-showing-as-read that you describe.
In the case of quitting the browser I believe the change from unread to read is the expected UBBT v7.x behavior. No idea why we're getting different results when reloading a page or closing a window though.
So now I have no way to tell what I've missed this morning short of combing all the forums for new posts.
If you have a general idea of when you last visited FTM, you could use the Active Posts option in the Active Topics menu in the navigation bar. Would be faster than checking each forum.