This is because It's not possible to read the settings from about:config with the new API. Therefore I'm using the default firefox zoom level settings.

edit:
What do you mean by "Custom zoom settings"? The "zoom step" is working fine for me, in case you are talking about that.

edit:
I agree, but I can't change it. It is limited by firefox likewise you can't zoom past 300% by pressing Ctrl and +