Hi Jonny,
Thanks for choosing Apex!
I visited the site in Firefox and was able to see the Featured Images. However, they did load quite slowly, so it’s possible the browser timed out the session before loading them. I would recommend resizing your images, and compressing them to speed up the site.
For instance, the blue image on this page is 4288x2848px and has a filesize of 4.4MB which is very large for loading on a website. I resized this image to 1200x797px and compressed it using TinyPNG. The filesize is now just 155KB and it will look just as good on the site.
Here’s a comparison of the current image and the one I resized/compressed:
Original Image: https://pics.competethemes.com/3T1D1M3t1j1u/image-original.jpg
New Image: https://pics.competethemes.com/2n292x0v0Q2G/image-new.jpg
If you resize and compress each of your Featured Images, your site will load way faster and the images will likely appear in Firefox again.