I've got some really weird behavior going on. I can't do any web searching using Google or Bing because when the search result page shows up, every single link on the page is completely unresponsive. That is, every link to a search result. The links to page functions other than search results work fine. This happens in both IE9 and FF13.
It doesn't happen to Yahoo! results, though.
I found the problem tonight. The actual issue extended even to Firefox not blocking ads any longer, and while on YouTube, an ad panel would appear just above YouTube's own top ad, and it would be telling me that I had several Windows errors (the number appeared to be generated at random), and there was a simulated dialog box inviting me to click on it for a solution. Exploratorily, I did click on it to see what happened, and was taken to some malware removal site -- that looked very very suspicious. I closed it without further action.
Here is what I found, after following the suggestion in the Accepted answer:
- after disabling add-ons in FF one by one, I discovered that the culprit was some Facebook downloader called, oddly enough, fbdownloader 1.0.4, by HTTO Group. I don't remember installing it. Anyway, when I disabled it, the ad panel appearing above YouTube now longer shows, and
- after disabling the add-on in FF, when I checked Google and Bing, suddenly I could click on search results. In BOTH FF and IE! Why would disabling an add-on in FF affect IE? Weird.
- When you check the HTTO Group's website, they claim that their fbdownloader product is certified 100% free of adware, spyware and virus. Bullpuckey.
Thanks for the accepted answer, which led me to the solution.