Not compatable
My lg-ms690 with android 2.2 os says that firefox is not compatable why
Alla svar (1)
Unfortunatelly your phone has Armv6 and Firefox Mobile is not supported by those phone.
Here is a try build: https://ftp.mozilla.org/pub/mozilla.org/firefox/try-builds/mh@glandium.org-a3d9309084af/try-android/fennec-13.0a1.en-US.android-arm.apk
This should work.