Hide number spinner in firefox browsers. (#8731)

I have added the css for hiding the spinner appearing on input type number specific to firefox browser.
Tested in Firefox 57.0 (64-bit).

Author: chetankhilosiya

src/material-examples/form-field-prefix-suffix/form-field-prefix-suffix-example.css

@@ -15,3 +15,7 @@ input.example-right-align::-webkit-outer-spin-button,
 input.example-right-align::-webkit-inner-spin-button {
   display: none;
 }
+
+input.example-right-align {
+  -moz-appearance: textfield;
+}