Please try to run firefox from a terminal to be able to see more output messages. It could be that the card is broken, I've seen cards where the basic graphics functions where fine, but they went crazy each time advanced graphics was used.
Here is my integrated video infos, and everything is working fine: $ dmesg | grep radeon radeondrm0 at pci0 dev 1 function 0 "ATI Radeon HD 7560D" rev 0x00 drm0 at radeondrm0 radeondrm0: msi radeondrm0: ARUBA radeondrm0: 1680x1050, 32bpp wsdisplay0 at radeondrm0 mux 1: console (std, vt100 emulation), using wskbd0
