black preview image - Firefox

[alexyProjet]

Describe the issue
I integrated nanogallery2 on my website https://cupote.fr
It works normally on Chrome but in firefox 115.0.1 macOS, the preview image are completly black most of the time (see screenshot) When clicking on the black image, the full picture loads fine.
This issue happens most of the time (maybe 80-90%)
https://ibb.co/nPvq7s9

CODE PLAYGROUND
Here is the code : https://codepen.io/tikuf56/pen/OJaEgdm
please don't mind the missuse of itemsBaseURL as it I tried this to solve the issue but it changes nothing

URL of your page
https://cupote.fr#gallery