In layers it is not possible as far as I know.
In the new 1.5 alphas, which have real tabs,
you can try to enable tab buttons in about:config,
modifying the line
browser.tabs.closeButtons
to the value 2, or adding such a preference line,
if it should be missing.
I can't check it at the moment, because I am
using Linux on this computer, and I have no 1.5 version
ready.
If it is not included, you can always ask Dorian
to try to do that in a later update.
Personally I do not think that this is an important feature,
because double-clicking the tab closes it it easily,
and having x-buttons on the narrow tab buttons would be
not such a beautiful thing to look at.
Heloo here. I allso think that the button is very usefull option because many users dont have habbit to press midle button or do double click.
It can be option, to chose would you like X button or not in browser preferencies.