Shouldn't pgns show up the same in both safari and ff?

So I have this blog, and I made a background, saved it as a png, and it shows up differently in safari and ff/camino.

Yes, I know I should save it as ‘for web and devices’ in photoshop. But this guy I know told me that this isn’t needed for pngs! Is this true? Any suggestions?

I’ll only post a link if anyone asks, I’m no spammer.