Rated 5 out of 5 stars

for those who prefer tweaking with userchrome, instead of extensions...

add the following code to userChrome.css : .

.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;
}

This review is for a previous version of the add-on (0.2.1).