I had this addon idea tonight and dont know if anyone has attempted it, but it seemed like a decent idea.
Basically when you click on a posted URL, it would be grabbed and stopped so it doesnt load right away.
Then a dialog would pop up with the site loaded into the WC3 HTML verifier
The addon would be scripted to search for specific malicious things such as:
mark them as found, in red, etc.. let you know if the site is safe
then you could click a button labeled VIEW SITE on the dialog to THEN go see the site...