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So, all is well, except that of course, the executable file in File Explorer still has the generic sheet of paper icon. I did a search online, found an icon for Thunderbird I could download, changed the desktop icon to that one, and that worked just fine. An additional tidbit that may be a clue to the problem is that if I go to the File Explorer (I think that's what they call Windows Explorer in Win 10), and look at the executable file there, it also has the generic paper sheet icon displayed. The generic sheet of paper icon was still there. I clicked the Change Icon option, found the original icon listed, chose it, clicked APPLY, then OK, and no dice.
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So I right clicked the icon, chose PROPERTIES, and the Properties box showed the original icon. However, the program still worked fine, and double clicking the generic icon started the program and it performed normally. It changed to the generic sheet of paper icon that Windows displays when it can't find the specified icon. At first, all was well, then, about the 2nd or 3rd day, the desktop icon for the program disappeared. I'm in the process of installing new applications, and downloaded and installed the current version of Mozilla Thunderbird to handle my e-mail, since I have that program on my old system, and am familiar with it. This has now happened twice, so can sort of no longer regard it as an isolated incident. But I have encountered a strange phenomenon. Please be patient with me, since I'm brand new to Windows 10 (my ancient Windows 7 machine is on its last legs, I fear), and am still learning my way around.
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