Hello Evert, first of all sorry for late answer. I am unable to check your implementation as your provided example link http://stlucas.wijzijnmerlin.nl/ is down.

For now I guess may be its your theme CSS or container where you puts the slider shortcode creating the problem. I see our demo sliders https://www.wpcue.com/wordpress-plugins/advanced-post-slider/template-one/ showing properly in firefox.

Please check our demo pages https://www.wpcue.com/wordpress-plugins/advanced-post-slider/template-one/ hopefully you will get help to implement properly.
If you need more help feel free to writing back.

Thank you