(Browser is Firefox 5.0)
If you go into Focus mode and then open a thread there in an alternate tab (so you have the Focused message boards on one tab and the thread you want to read in a second tab), and you clear a dot from the focused message boards, that tab will refresh into the open thread, so you end up with two tabs open to the same thread.
(No idea what would happen if you open multiple threads on multiple tabs...)