It seems to be happening on all the mozilla-based browsers on my system. Hopefully its just some rogue script on my machine. I've run ad aware but it didn't clear the problem. Virus checker has reported nothing unusual.
Using AdAware and SpyWare should get all spyware and adware installed on your computer. AdAware doesn't catch all, while spyware will catch what adaware misses. And visa-versa.