Here are a few ideas from the experts:
Firefox attempts to comply with all web standards. However, just like all other browsers, there are areas that do not comply.
Firefox has a basic built-in developer interface. Additionally, a number of extensions can be used to improve the developer support.
CSS are the way that documents appear. Both the document's author and the viewer can apply style sheets to modify the document's appearance.
Firefox uses CSS for the browser's look and feel and for content. All styles are created using CSS and images.
Chrome refers to how the program (Firefox) appears to the user. When a custom style is used, the Chrome is modified.
Content refers to the content displayed by Firefox. You can either accept the author's styles or apply your own styles to content.
CSS allows multiple levels of styles to be applied. Styles loaded later can take precedence over earlier styles.
You can use !important to signify a style that must be adhered to. Of course, the last loaded !important overrides earlier occurrences of !important.
The Firefox DOM Inspector allows viewing the document hierarchy and object properties.