A homepage is what appears when you launch the browser. A new tab page is the page that appears when you open a new tab. I want a custom new tab page, which is a feature Firefox disabled. I personally do not care if the homepage and the new tab page are different as I never use the homepage (I always choose to restore from previous session).
Oh, I had misunderstood. I thought from the way the comments were going that you wanted the opposite - that they allow you to customize New Tab but not the Homepage, and I was wondering who uses the home page anymore.
I skimmed through about:config and didn't find any alternate way to change the New Tab URL other than a browser extension, like you said.