I also have the problem of not being able to disable the top sites. I've tried the normal method, but there was no newtab entries in the about:config file. (That was a problem in itself, apparently having adblock plus installed prevented the about:config file from being opened, and I discovered this by going through and individually disabling the addons..)

So I thought that perhaps it simply needed to be entered, so I created two new strings, hoping this would solve the problem:

string => browser.newtab.url => about:blank boolean => browser.newtabpage.enabled => false

I then reset the phone to ensure that FF rebooted, but to my dismay that didn't solve the problem either.

I don't know that the following will be useful, but you never know, so here is the info about the phone:

Motorola Droid Razr, un-rooted, Android version 4.1.2 System version 98.72.16.xt912.verizon.en.us (this retarded update is causing me to look into rooting...) and FF19.0.2

thx gwystyl, I added the strings to the config too without any satisfactory result. btw I'm on 19.0.2