Have you tried the steps in the Basic Troubleshooting KB article?

Thank you. I didn't know about the resource you suggested. I followed the steps in the troubleshooting procedure, and found that the culprit was one of my recently added Add-ons, DoNotTrackPlus.

This add-on has the admirable goal of stopping advertising companies from tracking my surfing habits, and serving ads that supposedly are catered to my needs. However, on just one of my websites, it was blocking four very important companies, Google AdSense, Doubleclick, AddThis, and even Google Analytics.

This not only prevented the AddThis buttons from showing on my websites, it was also preventing my AdSense ads from showing - and probably preventing me from being tracked by Google Analytics! I don't so much mind these things being blocked from my own view, but I cringe at the thought of how many of the visitors to my websites are also being prevented from clicking my AddThis buttons or my AdSense ads!

I have written a very critical feedback to Abine.com, the authors of the add-on, asking them to please immediately remove the above four companies from their list of sites to block, and I hope that they see fit to do this ASAP. In the meantime, I hope this reply will help other webmasters who may be experiencing this bad behavior.

Awesome. Glad to hear you fixed it!