Code:
/* Add tab-close-button to last tab*/
.tabbrowser-tabs[closebuttons="alltabs"] >
.tabbrowser-tab > .tab-close-button {
display: -moz-box !important;
}
.tabbrowser-tabs:not([closebuttons="noclose"]):not([closebuttons="closeatend"]) >
.tabbrowser-tab[selected="true"] > .tab-close-button {
display: -moz-box !important;
}
That's a copy/paste of my whole userChrome.css, which has to be in the "chrome" folder of the profile you use.
My about:config settings are:
browser.tabs.closeButtons = 1
browser.tabs.closeWindowWithLastTab = false
That's at least what works in my installation. With those settings you should have the "close" cross, even when there's only one tab left and if you close the last tab, you should get a blank page.
Together with the NewTabURL add-on, you can let the FF open your starting page or any URL you name in the extension (which then is opened for any new tab; I chose my starting page, which is Speed Dial).
As I said, that's at least what works fine for me.