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This is a "very loud" in-your-face error which occurs just in Firefox. I don't yet know how to beat it. I found something online which suggested that if you pick "Restart with Add-ons Disabled..." under "Help" that it will run Firefox without any Add-ons so that you might rule out an Add-on as the culprit, but in my case I think the culprit is DXR.axd of DevExpress. Anyways, if you can't see the "Help" menu...

1. go the hot dog menu at the upper right
2. pick "Customize" at base of menu which flies out
3. pick "Show / Hide Toolbars" at lower right
4. pick "Menu Bar"