Note that anything starting with 'chrome://*/skin/' is provided by the current selected theme. There are not garantees what should by provided by that theme. The theme will generally provided at least the items referred to directly by the standard applications such as 'browser.css', 'menu.css', etc.

Any hard references to images should be prevented at all. Better is to use 'class' to style your items. If you use for example '.tabs-closebutton' you will get the same look at the standard tab closebutton...

So, not only there are differences between Mozilla Suite and Firefox, also between the standard themes, but also between other themes.

So, do not rely on 'chrome://*/skin/*.gif" etc...