Never turn off your firewall. Using an always-on connection without firewall protection can result in infection within minutes.
Have you given your browser full permissions in the firewall?
Double-click the taskbar icon to open SecurityCenter
Click Web and Email Protection, then click Firewall
Click Internet Connections for Programs and then click the word Program at the top to arrange in alphabetical order.
Locate your browser(s) and make sure their permssions are 'Full" - use the Edit button if need be.
In your browser(s) settings, make sure they are at their default.
Thank you for the reply Ex_Brit. I understand the implications of switching of the McAfee firewall but the problem of not being able to add attachments to Yahoo webmail messages still occurs when the McAfee Security Center tells me the Firewall is OFF. So, I had presumed I had eliminated the McAfee Firewall from being the problem.
Does this mean that it is not possible to switch off the McAfee Firewall and use the less sophisticated Windows one? (If so, this seems like a bug)
Is there another part of McAfee protection blocking communication with Yahoo? The problem goes away when McAfee is uninstalled.
SiteAdvisor perhaps? Is it installed - you'll see an icon at the top of the page? Clicking just to the right of it will bring up the options button. Make sure Protected Mode is off - if available on the version you have.
Other than that all I can think of is Netguard which is part of Firewall and can be turned on or off for each item in the permissions area or turned off altogether in the Web and Email section. try that. If it doesn't help I suggest opening a case with Technical Support. It's free via phone or online chat and is linked under Useful Links at the top of the page.