Good morning Marc and welcome to the Events Calendar Support forum!
Thank you for reaching out to us.
We are sorry to hear about the missing white space between rows with Firefox on your site.
I would love to help you with this topic.
As a first troubleshooting step, could you please provide us with your complete system information in a private reply using the instructions found in the following link?
https://theeventscalendar.com/knowledgebase/sharing-sys-info/
This way I will be able to take a look at your site and help you better.
Have a great day!
Geoff B.