`.weui-input` should hide spinners and arrows in firefox as well

[Ken Ho]

The current .weui-input style does not hide spinners and arrows in Firefox when input type is number.

weui/src/style/widget/weui-cell/weui-form/weui-form_common.less

Lines 35 to 40 in e7bf215

 // hides the spin-button 
 &::-webkit-outer-spin-button, 
 &::-webkit-inner-spin-button { 
     -webkit-appearance: none; 
     margin: 0; 
 }