We've opened a ticket with Sizmek and this is what they replied to us and then closed the ticket:
I am replying on behalf of Joey. I believe this only happens when a video is attached.
Please check with Adobe about the problem since there are similar issues with edge not loading on initial run in Firefox.
Did you find a solution to this issue?
I've found the same thing happens to me. I thought it was as issue with the browser itself and was hoping an update would fix the problem, but months later and it is still happening.
I have noticed if you refresh the page or refresh the ad in the sizmek panel, the ad and video will play.
First time around though, it certainly does hang and not load.
I've had the exact same problem. I have found that the banner plays as expected when run locally (although this could be due to caching), its only when run remotely on the Sizmek platform that the problem arises.
Something prevents the "compositionReady" event from firing...
...on further investigation; including an ogv version does seem to resolve this for some reason (FF should play mp4s without a problem).
The ogv needs to be first in the source order. I think this has to be done manually in the code.