In general there are two classes of CPUs in Android devices ARMv6 and ARMv7. Firefox requires ARMv7 devices. If you don't see Firefox in the Android market then you are likely on an ARMv6 device.

In the future we hope to provide a solution that allows Firefox to run on ARMv6.

I believe the LG P505 is basically the LG Optimus One under name of LG Phoenix. This phone has one of two 600 MHz Qualcomm cpus depending on gsm or cdma and are ARMv6.

http://en.wikipedia.org/wiki/LG_Optimus_One