I'm sorry you're experiencing issues working in Firefox. Are you finding the same issues when working in a different web browser? If so, you may need to check for an add-on or extension that needs disabled and/or clear the cache in Firefox. You'll also want to make sure you're working in the most recent version of the web browser.
In addition, please know that all customer feedback about our New Experience is being forwarded directly to our developers. Feel free to post your feedback about email campaign thumbnail previews in the Email Campaign Experience section.