Hope I can help you a bit... Don't worry, you don't need to spend any money..
Like Peddant said you can enable the Fx 1.5 popup blocker, to do this click on Tools>Options>Content, then make sure "Block Popup Windows" is checked.
That alone is not enough, now there are ways around the popup blocker, and many people are experiencing popups even with the built in popup blocker. The best way to get around this is to first download the extension Adblock Plus and open it with Fx to install it. You can get the latest version of Adblock Plus from here:
http://p2.forumforfree.com/adblock-plus-05112-released-vt352-adblockplus.html
Then get the Adblock Updater, the latest version can be found here:
https://addons.mozilla.org/extensions/moreinfo.php?id=1136
(Adblock Updater is also a Fx extension, so you will need to open it with Fx in order to install it.)
After the extensions are installed, close Fx and then open Fx. The extensions will now be installed. If the Adblock list isn't updated automatically, goto Tools>Adblock Plus>Filterset.G Updater>Settings and click the button force update now.
Also like kodrutz said, get NoScript, it is a very good extension that will block anyother security issues in Fx. Oh, and don't forget to download the latest version of Fx (1.5.0.1 RC1), I always stay with the most current version, but you may wish to stick with 1.5 which is probably the most stable build.
Fx 1.5.0.1 RC1 can be found here:
http://www.majorgeeks.com/Mozilla_Firefox_d2248.html
Further adblocking and popup blocking shouldn't be needed...
Norton's Clean Sweep is a good program, but try looking for freeware before you spend any money. Here is a list of downloads including Freeware and Shareware:
http://www.majorgeeks.com/downloads12.html
You can actually do all the cleaning that Norton's Clean Sweep does by yourself without a program, but it is a lot more work, so I suggest finding yourself a good freeware program to do it for you. If you need help with anything, just ask. Good luck.