[fix] Firefox rendering of TextInput

Removes UA controls from numeric inputs in Firefox, and makes the
default border solid in all browsers.

Fix #789

packages/react-native-web/src/exports/StyleSheet/StyleSheetValidation.js

@@ -93,8 +93,10 @@ StyleSheetValidation.addValidStylePropTypes({
   cursor: string,
   fill: string,
   float: oneOf(['left', 'none', 'right']),
-  font: string /* @private */,
   listStyle: string,
   pointerEvents: string,
-  tableLayout: string
+  tableLayout: string,
+  /* @private */
+  MozAppearance: string,
+  font: string
 });

packages/react-native-web/src/exports/TextInput/index.js

@@ -390,10 +390,12 @@ class TextInput extends Component<*> {
 
 const styles = StyleSheet.create({
   initial: {
+    MozAppearance: 'textfield',
     appearance: 'none',
     backgroundColor: 'transparent',
     borderColor: 'black',
     borderRadius: 0,
+    borderStyle: 'solid',
     borderWidth: 0,
     boxSizing: 'border-box',
     color: 'inherit',