A few weeks ago on my Minecraft server of choice, I made a thing:
It's about 50 by 50 blocks. How did I make this, you ask? I took the actual Firefox logo, resized it to 50x50 pixels in Gimp, and reduced the colors pretty drastically. Then it just boils down to putting a grid over the image and turning the pixels into blocks. (Luckily, the Minecraft server is in Creative Mode, so I could freely pick blocks according to their colors, even though some of them are really rare when mined.)