malicious toolkit web site 9 on firefox start

Hello,

I've recently been receiving "website attack malicious toolkit web site 9" warnings from Norton upon starting firefox.  It doesn't happen every time.  Also, when firefox starts, I only have trusted sites open, and normal browsing on these sites does not seem to generate any sort of warning.  I'm not sure what to do, but this is worrying, and I'd like advice on what I should do. 

 

Thanks.