See this - https://support.mozilla.org/en-US/kb/will-firefox-work-my-mobile-device?s=mobile+compatibility&r=2&as=s#os=android&browser=m14

The LG LS670 Optimus S (from Nov 2010) has a older ARMv6 type of cpu (a Qualcomm MSM7627) when a ARMv7 is required.

Mozilla is looking to have ARMv6 builds of mobile Firefox releases in future, though while the phone has 512MB of Ram (minimum recommended), the cpu is only 600 MHz (when 800Mhz is recommended).

You could in mean time try these unofficial third-party ARMv6 builds at your own risk at http://forum.xda-developers.com/showthread.php?p=25855563

https://support.mozilla.org/en-US/kb/will-firefox-work-my-mobile-device?s=mobile+compatibility&r=2&as=s#os=android&browser=m14

It incorrectly lists the LG LS670 Optimus S as being compatible as it is removed in a recent revision that is Unreviewed.