Firefox is known to cause a few display issues in certain pages of the application. Our general recommendation is to use Chrome.
I'm not sure that issues specific to Firefox for the ACS content editor will be fixed since the team is currently working on a major update for this component that we should see coming in the first months of 2018.
It is still in development phase so I won't share much here for now. What I can say is that it will be way easier to design emails from scratch, or to edit existing/imported emails if you need to add/remove content.
For sure, I will share more on that topic when the time comes .
Thank you for a fast reply. I've tested in incognito and it made no difference.
The problems occurs only in Firefox, since I've updated to the most recent version. I'm running on a windows 10 computer. Yesterday, I tried from my Macbook in the new Firefox browser, which worked perfectly - so it is an isolated issue.