block sites?

Hi. I keep receiving pop-ups from only a few certain sites (which I do not remember visiting ever...they're not vulgar, but they ARE annoying... ex: travel agencies, "free" offers, etc.) Anyhow, I am using Mozilla Firefox (and before you tell me to switch browsers, please be aware that I'm not going to, sorry!) is there any way to block a certain site? I know that Internet Explorer had something like that, but I can't find it on Firefox. Any help would be appreiciated. Thanks.

Answers:
If you're getting random pop-up ads you may have some spyware installed. Download one of the many free AntiSpyware programs.

Microsoft has one
Spybot is good, too

either visit the Microsoft site or www.download.com and search for antispyware.