I actually found the solution myself.
In your url field type: resource://gre-resources/forms.css
- this will open forms.css for FF4 - and search for the following selector
button::-moz-focus-inner,
input[type="reset"]::-moz-focus-inner,
input[type="button"]::-moz-focus-inner,
input[type="submit"]::-moz-focus-inner,
input[type="file"] > input[type="button"]::-moz-focus-inner
Now just put that in your main stylesheet like:
button::-moz-focus-inner,
input[type="reset"]::-moz-focus-inner,
input[type="button"]::-moz-focus-inner,
input[type="submit"]::-moz-focus-inner,
input[type="file"] > input[type="button"]::-moz-focus-inner {border:0;padding:0;margin:0;}
It shouldn't be overriding your css anymore. Also note that if you haven't defined any font style for your inputs FF won't inherit body font styling.
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