For me it works like this:
Go to main page (i log in at forum.ecomodder.com not ecomodder.com/forum)
go to sub group.
go to post & read it (any and all new posts).
go back to top of page and click on
"Ecomodder.com Fuel Economy Forum"
http://forum.ecomodder.com/
i'm back at the main page. the sub group says it has unread threads.
click on subgroup - no unread threads are there.
go back to page and click on
"Ecomodder.com Fuel Economy Forum"
http://forum.ecomodder.com/
again
sub group is now marked as totally read.
i'm using firefox 2.0.0.11