There might be a workaround for you...
In your file, I set the expanded page to be the primary page and uncheck the Expanded checkbox in PI. Then on the banner page, I check the Expanded checkbox in PI
We have built expandable banner in GWD but it does not appears properly on Mozilla Firefox.
Banner when loaded on Mozilla FF appears a blank page with a close button before actual display. Below attach is the screenshot for your reference. Kindly let us know the issue.