Video Player is not working on firefox

I have the same problem. I checked and it works fine on IE (that's surprising) and Opera. In Firefox and Chrome, the player is here but invisible. It has his place, the cursor changes when you hover and when you click, it starts playing, the player and the video are not visible.

Actually, by the time I checked my page to copy-paste the url here, I works ! But not entirely, some content have not the player displayed yet ; it's weird...
[edit] It's even weirdest because one page can work, the player is visible, then I refresh the page and the player disapear again...

here it's working now : http://www.rcdevsite.be/testimonials/bayer/wim-gerits
but here it doesn't (yet?) : http://www.rcdevsite.be/testimonials --> it's a view
[edit] I have changed to JWplayer 5.4 so you can't see the problem any more

And It's JWplayer 6 ; I put JWlayer 5.4 and it's working fine.
And also, I have Drupal 7

I believe there are some computer systems / graphics cards, that are not playing nice with firefox/flash. I'm in that boat myself. I have a 30" monitor and a Radeon graphics card. Recent firefox updates have not solved my inability to play flash, whereas other browsers play flash just fine on my system. So while JWPlayer might offer an alternative (such as recognizing if Flash fails and serving up the video via an alternate method), I think the bug is between the graphics driver and the browser-specific flash plugin.

I'm hoping for a workaround for this and for iPad -- interested in comparable solution to JWplayer that does not rely on Flash.

We're not seeing any issues with Firefox and JW Player 7.x-2.x.

Please download a copy of 7.x-2.x-dev and add these patches to review. We'd appreciate any feedback you might have. Thanks.

* Refactor theming: https://www.drupal.org/node/2713725
* Add not empty checks and Repeat option: https://www.drupal.org/node/2719995
* Add js file to build player: https://www.drupal.org/node/2719977