Learn basic HTML/CSS. I mean, really basic. Like changing colors and links and stuff.
Download someone else's homepage and save it as a text file and a .htm or .html.
Edit the text file to whatever you want. Change images, colors, links, whatever.
Google "firefox change new tab," and change your new tab to the .htm saved on your computer.
These links are basically everything I used: http://ayychloe.com/guide http://neiio.deviantart.com/art/How-to-Install-Custom-Themes-262833454