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Firefox Quantum keeps asking me to close Firefox on external launch

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When launched directly from Explorer, Firefox Quantum (v62.0.3) works fine. But when launched indirectly, say via some other program sending a URL to the default browser (FF), FF launches first but then it always brings up a dialog box saying that I have to close FF first! When I click the dialog button to do that, the existing FF window is closed, then it re-launches with the requested URL.

I'm running 64-bit FF under64-bit Windows 10 Pro.

What's going on, and how can I fix this extremely annoying problem?

Thanks!

When launched directly from Explorer, Firefox Quantum (v62.0.3) works fine. But when launched indirectly, say via some other program sending a URL to the default browser (FF), FF launches first but then it always brings up a dialog box saying that I have to close FF first! When I click the dialog button to do that, the existing FF window is closed, then it re-launches with the requested URL. I'm running 64-bit FF under64-bit Windows 10 Pro. What's going on, and how can I fix this extremely annoying problem? Thanks!

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Added Info: Here is the text of the dialog pop-up in question:

"Firefox is already running, but is not responding. The old Firefox process must be closed to open a new window."