Adblock works just fine for me. Try using the NoScript extension, it blocks all of the side ads on Geocities as well as annoying things like the sliding menu bars on About.com.
Alot of ads are Flash based. The Flaskblock extension is great.
You can also use a HOSTS file
just like windows. Or a proxy like Privoxy
can be used to block ads also.
You can also download a block list for your hostperm.1 file that will block cookies and images from alot of bad sites.
A list of ad servers and how to use them to easily block 90% of ads on the web.