LG Vortex aka LG Optimus depending on variant or carrier. I think it may be using the Qualcomm MSM7227 which is a ARMv6 and part of ARM11 family. That is the problem as the current Firefox mobile release supports the ARMv7 chips. If it is not in Android Market on the phone then that is because the hardware does not support it yet. You could try the Fennec nightlies at https://wiki.mozilla.org/Mobile/Platforms/Android#ARMv6_.28experimental.29