Issues with tooltips not working properly have been reported to be caused by the Google Toolbar (Tools > Add-ons > Extensions)
You can check if there is an update available that fixes that issue.

You can try to uninstall and reinstall the latest Google Toolbar.

• https://www.google.com/intl/en/toolbar/ff/

I don't use the Google Tool Bar.

Your More system details list shows a user agent with the Google Toolbar.

• Mozilla/5.0 (Windows; U; Windows NT 6.1; nl; rv:1.9.2.15) Gecko/20110303 Firefox/3.6.15 GTB7.1

See:

• Troubleshoot extensions, themes and hardware acceleration issues to solve common Firefox problems