Chrome does not allow styling form validation bubbles anymore: https://code.google.com/p/chromium/issues/detail?id=259050
Additionally, Firefox has support for the element attribute x-moz-errormessage which enables you to change the text of the error message, which is something you could do in Chrome using CSS to and -webkit-validation-bubble-message. See more about x-moz-errormessage on the MDN Docs page.
As of yet, Firefox has no way to style the error bubbles.