'Hide Tab Close Button' preference disables the button but still draws icon.
By shab... on September 11, 2014 19:57 (imported from Google Code)
Using Build 2.9.20140904-nightly on 10.9.4
Enabling the 'hide tab close button' preference stops the [X} icon on the tab from actually responding to clicks, but it is still drawn.
Disabling the option makes the close button work as expected.
Bit of a faff to generate config/debug dumps just now, but can be provided if necessary to repro/trace.