Firefox has slowed down after installing anti-virus

Hello,
My Mozilla Firefox browser has slowed down after installing avast free anti-virus. This has caused me hamper in my job. I tried re-installing it did not seem to work. I have just installed the anti-virus and I do not think uninstalling it again would be a good idea. please someone tell me what to do. It will be better for me if the problem is solved quickly.
Frank Marlow
