Note that Firebug only works within the browser. The changes you make to the CSS within Firebug are not permanent. If you want the changes to be permanent, you need to change the files on the server. There are two extensions named
FireFile and
cssUpdater for Firebug, which aim to synchronize the changes you've done in Firebug with the server files. Though those extensions are not maintianed anymore, so they might not work anymore. So you'll need to do the changes directly on the server files.
I didn't try out out Zen Cart so far, though you should find the related information in the Zen Cart wiki:
http://www.zen-cart.com/wiki/index.php/Customisation_-_Templates_-_StylesheetsSebastian