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!