Even extensions in Firefox can't customise the page you see when you open the browser! I used a "new tab" extension, but for some reason Firefox doesn't treat the first tab you open as a "new tab"(1). I quickly noticed heaps of annoyances like that when I recently tried to switch to Firefox.

1. I'm pretty sure there is an issue for this in their tracker but they don't seem to think it's important enough to fix.