This is a common problem with ff.net caused mostly by their advertisers. You can fix it to an extent by replacing your computer's "hosts" file, which tells it not to open content from certain sites. To do this go to:
http://www.mvps.org/winhelp2002/hosts.htm go to the hosts.zip link, and follow the directions for the windows program you are using.
There's a batch file that automates the installation further down the page; you can also download a program called KH Blocker that automatically updates your host file every day, but it's a bit of a nuisance because it pops up every day and tells you what it's doing.
