I’ve found Mozilla app icons have disappeared when I open Thunderbird (email) and Firefox (web browser) in Windows 10.
The fix was quite simple in the end.
- Open the start menu programs folder
- RH-click the application shortcut and choose Properties > Shortcut > Change Icon.
- Select your icon and click OK > OK
It may take a couple of goes to get this right. I found two icons for Thunderbird, the first renders as a blank document in the Taskbar, but the second displays correctly.
And if that doesn’t work…
I also tried these instructions to fix the taskbar. They didn’t work in this case, but may help you.