TechMansor,
my comments to DistinctNews and FMacastro had to do with modifications to CSS styles for various elements. If you are able to find the style, and edit it in the style.css, then you know what element, class and style you are working with. And thus you should know what to put into the User CSS field. Mapping these classnames to what you actually see on screen can be challenging, but will get easier as you familiarize yourself with your chosen theme.
If you find a modification that you want to apply to one of the Mystique .php files, here is my suggestion:
Navigate to the Appearance - Editor section within wordpress admin. So if instructions tell you to edit header.php, find that file in the list to the right. The file will be shown in the Edit Themes dialogue.
Edit the file directly in there, and press the "Update File" button when done. Or, you can copy the text and paste into your favorite php-editor (for example notepad) and edit there and then copy-paste it back before pressing Update.
Alternatively you may want to use an approach where you logon to your server and do the edits there. That way backup is easier. Explaining that workflow is out of scope here...
The reason firebug seemingly does not help (it really do, just not the way you seem to expect), is that it only work with the html, css and scripts that the browser see when you call a page. It does not have the ability to tell you which PHP file(s) generated that html etc.
That is where the recipe comes in:
If do you use google (as I sarcasticly suggested), you might decide to follow the recipe on this link:
http://keremerkan.net/posts/adding-facebook-and-linkedin-icons-on-mystique-v2-x/
It actually does not require modifications to the core PHP files at all. It is all done in the mystique settings "User CSS" and Advanced - "User Functions" input dialogues.
Follow it step by step, and you should have your facebook icon in place. If you can't get it to work, you might want to leave a comment on that page?
Hope this helps...
PS: As for how things are done in other themes: I don't think that belongs here, as this thread was specifically created for mystique.