Sorry for the inconvenience caused, please clarify the following:
1. What type of internet connection are you using? (e.g: DSL, Cable, Fiber Optic etc…)
2. Do you have any other anti-virus/ security applications installed in your computer now?
3. Are you having issues only with Fire Fox or even with Internet Explorer?
If you installed the mcafee try restarting the computer in safemode with networking wther you are able to go online
First of all make sure that Firefox and Windows are all up to date.
Double-click the taskbar icon to open SecurityCenter
Click Web and Email Protection
Click Program Permissions
Make sure that both Firefox and Internet Explorer have full access, use the Edit button if necessary.
Depending on what operating system and service pack this is you could try repairing the internet connection. The method varies slightly from system to system.