This type of error is usually caused by an add-on you've installed. It could be anything from an antivirus link scanner like AVG Safe Search, or McAfee Site Advisor, or something similar.

Start Firefox in Diagnose Firefox issues using Troubleshoot Mode. If it functions properly in that configuration, then one of your add-ons is the culprit.

See also Troubleshoot extensions, themes and hardware acceleration issues to solve common Firefox problems and Troubleshoot issues with plugins like Flash or Java to fix common Firefox problems

I had the same problem . I disables ask.com in my tools and it took it away i do not receive the message any more.