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Go to the Firefox app

Select it and choose "Get Info"

Click on the Firefox icon that appears below the Red/Yellow/Green traffic buttons

Copy it using Command+C

Open "Preview"

Click File > New From Clipboard

Click Tools > Adjust Size

Change the scale until you think it looks good

Save as blah.png

In the "Get Info" window from earlier drag the blah.png file onto that Firefox icon

Profit!
 
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