As there is no preview button in the quick reply in XenForo 1, the full-size editor page behind the "More Options..." button serves for that purpose. If you're tempted to rename the button to "Preview...", the next question you will ask is: can I load the preview automatically on the next page?
With this simple add-on, you can. Thumbs up.
Or you cannot. Because it's not working with Firefox. But there's a solution to that, check the discussion thread.
An alternative to this approach is the preview overlay: https://xenforo.com/community/resources/quick-reply-preview.756/