My wife noticed that she got a message from McAfee (unremembered) just before the problem started, and that a green McAfyestee search window appeared on her yahoo e-mail account shortly thereafter.
Go to Solution.
Check McAfee setting to make sure that it is not blocking Firefox. For details see either Configuring McAfee Internet Security or configuring McAfee Total Protection.
First Solution refers to McAfee Security Center, but only item under McAfee is Mc... Virus Plus; 2nd reference is to a blank page.
Check the McAfee AntiVirus plus settings, the approach should be similar to that for McAfee Internet Security to see if it has settings to block applications.
The following link from McAfee may help - http://service.mcafee.com/FAQDocument.aspx?lc=1033&id=TS100810
You can also try checking with McAfee support about how to unblock programs using their products - http://www.mcafee.com/us/support/