Virus affecting only Firefox

[ControlZ]

My system running XP has become infected with a virus/worm/malware that seems to have only affected Firefox 3.5. What happens is that when I use Google from within Firefox to perform a search any result I click on is redirected to an IP address which takes me to a site containing a trojan or more spyware.

I have tired performing searches from within IE7, Safari for PC and Chrome, using Google, all of which work fine. I have noticed that whatever has infected my system is setting a cookie which is directly related to the sites I am being redirected to. If I delete the cookie I can use Google's search results, but when I return to Google, a new cookie has been set and the search results once again lead me to pages containing viruses.

I have performed scans using Malwarebytes, AVG, AdAware, RootRepeal and ComboFix. I did discover several viruses and trojans, all of which I removed from scans run in safemode. Subsequent scans show no new infections.

Does anyone know what virus may be causing this and what software I could use to try and remove this from my system? I forgot to mention I do have system restore disabled. One possible solution I had was to uninstall Firefox, including the settings it leaves behind and then try a fresh install.

[ControlZ]

After posting this message I for some reason decided to check my Firefox extensions and found that there was an extension I never installed. XUL cache 1.0

Once I removed this extension searches I performed on Google were no longer redirected to strange IP addresses.

Hope this help anyone who may come across a similar issue.