I have one particularly annoying/persistent popup that keeps coming through no matter what I try, including editing HOSTS and 2 different Mozilla adblock/scriptblock addons. Is there any way I can specify to NEVER open a given URL no matter what?
Sounds like you have some malware infecting your computer, best place to get help with that is the forum run by hijackthis, however if you use a router you can block the domain. This won’t prevent malware from attempting to open the link, but will prevent it from connecting until you can find and fix the cause. In other words you’ll probably still get the popup, it will just have an error message that the webpage isn’t available.