It's not uncommon for me to download numerous tools over a period of time while I'm online. Today I had spent the last few hours reading blogs, checking twitter, downloading a few SEO tools, and so forth. All of a sudden I notice that all of my links are highlighted in red!
Annoyed, because it was happening on every page no matter where I went, I did a little research and found the answer: The SEO toolbar I downloaded defaulted the setting that had caused this highlighting occurrence. If this has happened to you, the fix is simple:
Go to Tools/SEO for Firefox/Options. The setting is 'nofollow links.' Click the box "enable/disable highlighting." Refresh your page, and the links should go back to normal.