You need 512 MB of ram space at this moment, in the upcoming weeks, that should be tuned down to 384 MB.

This is a older entry level phone and has a 832 MHz Broadcom BCM21553 which is a ARMv6 cpu when ARMv7 is best and a small 2.6 " display at 320×240.

Hard to be sure but I believe this phone has only 384 MB of RAM.

It was only as of mobile Firefox 17.0 that ARMv6 cpu with 512+MB RAM was supported. For Firefox 20.0 the minimums will be lowered to 384+MB of RAM and QVGA displays which I'm guessing will be 320×240 which this phone has.

So you might be able to use mobile Firefox as of version 20.0

If you cannot wait, try and see if you can install the mobile Firefox Beta on Google Play.

Ok, so the RAM is the problem. I guess I'll have to wait to decrease the minimum amount of RAM required, as the change of the phone is not a solution for me right now :) Thanks guys!