Appear in FF when clicking Links with a negative margin (eg Img Repl Headings)
In your CSS document, just start out by putting this universal rule for all links:
a {outline: none;}
Browser-specific CSS hacks have become a taboo among standards-aware web designers for good reason; ideally you shouldn't need them. However, as long as the Internet Explorer 6 browser continues to hold significant market share, there will likely remain some cases where you need to use unfashionable CSS rules to accommodate outdated technology.