For some time I've taken advantage of the Style Sheet pop-up menu in the Advanced tab of Safari's preferences to load a customized cascading style sheet (.css) for, well, doing stuff that Safari doesn't do on its own.
I've been playing around with the Firefox browser lately and wanted the use this same style sheet with it, yet Firefox offers no obvious place for loading such files. After digging around, I found the answer.
To use a .css file with Firefox, follow this path:
Place the .css file inside the chrome folder and rename it userContent.css. When you next fire up Firefox, the settings within the .css file will take hold and perform their intended job.