It's purely the visual component I mentioned, the functionality seems to be okay, although it's reported as broken in the add-on manager. I have the tab bar docked on the left vertically and the tabs are ~3x taller than they should be.
I'm the same way about switching to Chrome, by the way. I would love to but I just can't picture life without TST after so many years.
I don't see the tabs taking up extra vertical space, although 3x the size should absolutely be noticeable. I did, however, have a problem yesterday when watching a fullscreen video: if I put my cursor on the left side of the screen, the tab bar would appear (as though I had it set to auto-hide) and obscure that portion of the video. It was quite annoying because the sensitive areas extended all of the way to the top left of my screen, where I have a "corner" set up on my Mac to disable the screen saver.
Ah, you know what, I just figured out what the issue was. I was using the "metal" TST skin and when I switched it to another one, the 150px-tall-tab problem went away.
I'm the same way about switching to Chrome, by the way. I would love to but I just can't picture life without TST after so many years.