You can use this code (in
Stylish or
UserChrome.css). It will 'faviconize' all tabs though, not selectively on your request (of course, change size to your liking):
.tabbrowser-tab
{ min-width: 26px !important;
width: 26px !important; }
.tabbrowser-tab[selected="true"],
.tabbrowser-tab:hover
{ min-width: 250px !important; }
Or you could pin tabs selectively from the Rclick on tab menu. But pinned tabs can't be closed, i think. I don't use it and don't really know. The close button can be displayed by a style as well but i'd need onemen' help with this one 'cause there 2 close buttons, left and right.
EDIT: JFYI, i'm not a developer and don't know what onemen will say to your request, this code is just a suggestion, tested on Fx15b2 and it worked for me.