Norton Safe Web, which was what I mentioned, is a web page you go to and enter the address you want to check to see if they assess it as safe (just like the McAfee link I gave). It's not a plug-in. Firefox can update and break many plug-ins, but either site to check to see if a specific page is safe will still work.
Here are the links again:
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Microsoft MVP/Consumer Security 2009-2010