I'm testing my html using CSS3. When I drop my .html file on to my Safari browser everything works, but in Firefox I get plain html with no CSS

Can you post the problem page online?

If this page is only on your computer, are you testing using a file:// URL? If so, is the CSS or JS in an external file? Firefox imposes certain security restrictions on paths when using file:// to avoid pages having access to folders beyond the "site" you're testing.

You could check for errors in the Browser Console (which you can open from the Developer menu). I suggest clicking the Clear button in the console, then reloading your page to see just the messages relevant to its initial load.

It is best to use only relative paths to additional files if you test locally and only go up to nested directories and not go back in the directory tree.