Warning unresponsive Script

This warning message is driving an old Irishman crazy.

I'm running win 7/64 and Firefox 30.

First occurrence was while composing an email message in Yahoo Mail three days ago, rendering Yahoo Mail totally worthless.

My attempt at fixing the problem, all to no avail, were:

1. Per instructions from Firefox: Disabled all plugins
2. Install a newer version of FF (actually, no newer version was found, reinstalled same version again hoping to overwrite any potentially corrupt FF files.
3. Ran MalwareBytes after downloading latest version.
4. Ran CHKDSK /F to repair any corrupt system files.
5. Ran a thorough boot scan using Avast (free version).
6. Ran CCleaner (free version).
7. In total frustration, I changed browser to Chrome.

None of these actions eliminated the warning popups.

PS: Not certain if this is related but Skype crashed days earlier and popped up: "skype.exe is corrupted!" Then Skype hung up tight forcing a system reboot. Skype hasn't crashed since.

PS 2: An attempt to Restore To Earlier Date failed.

Help me, please.