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Move the cursor to the Results pane, and right-click the desired application.
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Select Properties.
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On the General tab, click Change Icon.
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Select the desired icon, or browse to another location to select the icon. After you've selected the icon, click OK. The new icon appears in the Results pane.
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Right click on Firefox in the applications tab of finder.
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Click get info.
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Drag the .icns of which ever variant you want over the small icon in the top left of the get info window.
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You may have to remove Firefox from your dock/desktop and then drag it out from the applications folder and to the dock/desktop again.
Source - https://github.com/seirin-blu/Custom-Firefox-Logos
THE FIREFOX LOGO IS COPYRIGHTED BY MOZILLA. THESE IMAGES ARE TO BE USED ONLY FOR PERSONAL USE AND NOT SOLD UNDER ANY CIRCUMSTANCES EXCEPT BY MOZILLA. I DO NOT TAKE CREDIT FOR THE CREATION OF THE FIREFOX LOGO, ONLY FOR THE ALTERATIONS I HAVE MADE TO THE BASE LOGO. I CLAIM NO COPYRIGHT OR OTHER RIGHTS BEYOND USE OF THE IMAGE FOR MY DESKTOP ICON.