Rated 3 out of 5 stars
Testing on FF 9.0a1 (nightly at this moment), after manually editing FF maxversion parameter into install.rdf file.
I haven't observed that JS popup the 2 previuous posters described.
What I've observed is that if I don't check "Focus left tab when closing current tab" option, to focus the right tab on closing, this behavior is not always achieved. If I close tabs quickly with keyboard shortcut, sometimes I'm closing a left tab and not always a right tab, it's like it takes some time to change focus... and it's very annoying. Also, I'm not usign TabsGroup Manager that may have interfered with this option.
Now I'm using Tabs Open Relative (Modified) which perfectly fit my needs.