You have just discovered why all developers need to use source control (Git, Mercurial, Subversion, etc.)
That said, this isn't really a programming question and doesn't fit well with Stack Overflow's guidelines. The latest version of FireFox (the only one I've checked) lets you choose "Web Developer" from the menu, then click on the "Style Editor" choice on the top of the newly opened area. In there, you can see the css resources it is using.