If you select the element on the page you're interested in, FireBug will highlight the HTML and CSS that controls that part of the page. The CSS that is displayed by Firebug is the same that is in the CSS files or embedded within the page's HTML, but the HTML displayed isn't necessarily what is in the PHP files because PHP creates HTML on the fly, so if you go looking for it, you're unlikely to find it, it depends on how that part of the page has been created.
As an example, you might want to look for the bit of the page where Firebug displays the bit of a page that says "Stock: 17". You can find this in the page's HTML, but if you look at the PHP file you won't find it because it isn't stored like that. The part of the PHP page that displays this part of the page could be something like
PHP Code:
echo PROD_QUANT . $quant;
which to the uninitiated, bears no resemblance to what is sent to the browser.
In short, you can use FireBug to locate the CSS, but if you try to locate where an element is in PHP, you're likely to give yourself a headache as what you see doesn't exist in the file in the same format.