Recently, Google ads stopped appearing on my Firefox browser -- I think it had something to do with updating to Firefox 2.0.0.5, the latest security fix. I could see the ads of my sites and from other sites on my Opera browser but couldn't see them on my updated Firefox browser.

Searching for the solution on Google, I learned that this is a common occurrence after Firefox updates. The solution is to clear the cache of your Firefox browser. Go to Tools>Clear Private Data and choose Cache. Click OK. 

After this procedure, the Google ads started showing again. Hope this helps , especially webmasters.