Hi! I struggled with this one for some time, mainly because (and I read that this happened to others too, so it must be how the newest version of Mozilla works) I simply couldn't inject JS code. It didn't do anything when I tried, despite what I tried, it didn't work.
I finally managed to complete this level, since I understood what needed to be done, but by using Built-in Mozilla Delevoper tools. I wanted to share this, in case anyone is having difficulties like I did, I hope I'm not spoiling!
The people who got it by injecting, did you do it with Chrome?