Firefox has a bug where some images with transparency are not supported. We have reached out to Mozilla regarding the issue.
in the meantime, as a work-around please add your logos through the Brand Manager (available via Spark Premium Features) as follows:
- On the Adobe Spark Project page, click on "Manage brand" on the top left-hand side
- Click on the Logo icon to bring up the Logo panel (should be a thumbnail of your current primary logo)
- Select the Add "+" button at the bottom of the list
- In the File Browser window that appears, select the image you would like to upload