Sometimes video is not playing (in some browsers)

I am using PC with Windows 7 and Adobe Animate CC Vers. 18

Formerly we used Flash to create animations. Now we change these projects to html5 to implement them in our LMS. To recreate the old flash animations I used 3ds Max with its particle system and created mp4 video files.

1. First project: on the left side I have a button to start the video (right side of the stage). No problem.
   Everything works fine.
2. Second project: same like project one
3. Third project: Here I have two videos. On the beginning the first Frame of the video is shown.
   When pressing the PLAY button the video plays. At the end an image apears in front of the
   video. Clicking it the second video plays (same size and position like video1).
   Chrome browser => works like described
   Firefox / IE 11    => when starting the website no first video to play; it starts with video2

I have no more ideas where to look for whatever error. Even if I comment out everything with
access to video2 in my script it does not show video1 but the first frame of video2.