we have the same issue, contacted dbck support and they replied, the solution worked for us!, I hope works for you too.
Regards,
Jonathan
Creative Ad Support reply:
The issue is Firefox is caused by the "transform-style" value: GWD sets this value as "preserve-3d" in the page container which it doesn't cause any conflict in Chrome or Safari, but for some reason Firefox renders the images on top of the Tap Area or any other clickable area in the creative when the transform-style is set in this way.
To workaround this issue you can to do the following:
1. Go to the Code View in GWD
2. Look for this CSS class in the code: .gwd-page-content
3. Set the "transform-style" as "flat"
4. Republish your creative.