Richard,
I tried to reproduce your issue, even considering that I have a different theme.
I, always, start with at least two tabs, that being my Home group. I cut down to a single tab, and it looked normal, but if I unchecked the Always show the Tab Bar in the Options|Tabs there was no single tab to be seen. However, if I opened another tab, two tabs showed, both with the X. From what you describe, it does not appear that the situation is the same, because, if I understand correctly you see the single tab, but not the x, without the add-on.
I, still, fail to understand why you are operating the way I understand you do. In my case, if I click on the single tab X, I close the browser.
I may, eventually, start using the other theme, but for now, I still use both versions, although 3.5 is trying its darnest to elbow its way into the default install location, and take away default from 3.0.11. That is annoying!
UserAgent: Mozilla/5.0 (Windows; U; Windows NT 5.1; en-US; rv:1.9.1) Gecko/20090624 Firefox/3.5