This method does not truly alter the icon, just the appearance on the docker/taskbar. The advantage of this is you won't need to re-patch every-time there is an update. This is only going to work if Waterfox is pinned.
So let's move on to fixing this eyesore:
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Download old icon from here: https://github.com/MrAlex94/Waterfox/blob/fddc309b229add649254959a174b7ee0f54e0a1a/browser/branding/unofficial/firefox64.ico?raw=true
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Right-click Waterfox on the taskbar, Right-click Waterfox in the pop-up menu and choose 'Properties'
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Click 'Change Icon' and choose your downloaded old icon. Restart your PC.
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If the taskbar hasn't updated after the restart, Windows is holding the old icon in a cache. Download Winaero Tweaker ( https://winaero.com/comment.php?comment.news.1836 ) Run it, and choose 'Reset Icon Cache' from the left menu and click the Reset Icon Cache button.
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All done! The icon no longer looks like Woolworths.