Board Thread:Support Requests - Getting Technical/@comment-5466786-20130827193618/@comment-11733175-20131108094141

Unfortunately the text gradient only works in webkit browsers. This includes Chrome and Safari or any other browser that uses the Chromium engine. Browsers such as Firefox, Internet Explorer or Opera do not currently support this property, and it will likely be a few years before any of them do so.

There is no way to make the gradients work in Firefox, so you'll be forced to use the method you've used as an example in your original post.