I tried this and here's what I came up with:
I've tried to explain what I did below, but I don't think I did a very good job:
Spoiler for steps:
This is the tutorial (and video) I used. You will need the "Resource Hacker" program, but you don't need the "ITweakU" program the text mentions.
http://www.wikihow.com/Change-Firefox-Icons
This is the icon I used:
http://rocketdock.com/addon/icons/9232
It needs to be converted to a .ico file, which can be done here:
http://image.online-convert.com/convert-to-ico
Then run Resource Hacker as administrator, open firefox.exe, expand the "Icon" folder and then the "1" folder, then right-click on "1033", click "replace resource", click "Open file with new icon", then open your Holo icon. Then click "replace".
Now you have to go through all the numbered subfolders under "Icon", click on the "1033" inside each one, and make sure that there aren't any icons that have only the original Firefox logo. If there are, right click on the "1033" and replace as above.
I left the ones with the Firefox logo on the sheet of paper alone, but you can replace those too if you want.
Once you've gone through all the steps, save firefox.exe. You might need to reboot to make it change.
Unfortunately, as much as I love Holo and S&W, I don't think she'll be able to separate me from my beloved Chrome...
