On my Thunderbird Portable 60.9.1 32bit on Windows 10 Home 64 bit, one of my main folder icons has changed from a standard "folder" icon (Yellow tab) to one like that of "Sent" or "Trash" (blue square). The sub-folders have the correct icon.
Go to Help >> Restart with Add-Ons Disabled
Then see if all the icons are as they should. If they are then you know that it is one of those Add-Ons that is the main culprit as is usually the case.
I assumed that this is only in Thunderbird that the icons are changed and not in Windows wide folder properties. If it is Windows Wide then there is another solution but for that you'll need to go to Windows 10 newsgroup or Mozilla.General.
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