Umm...to get rid of Advertisements goto Tools>Add-Ons>Get Extensions download NoScript or Ad Blocker Plus.
If you use NoScript, you will need to "allow" each web page you visit using the options menu on the bottom.
I hope I've helped your situation. Next time you have a question, be more specific (stop using the word "thing" for what you really mean). I can only assume that you meant to say, "I am having trouble with Mozilla Firefox v.3.0.0. Everytime I try to type in a new web address a pop-up diverts the whole process. I would like an extension/Add-on to prevent pop-ups."