Seems to work fine for me in Firefox Dev edition (84.0b5), but in standard Firefox (83) it's generates the issues discussed.
You can see this on our current development site here:
Link to site removed
Everytime the page is refreshed the underlying correct slices show for a second and then "your slice is empty" shows.. and the page seems broken to the user.
It's feeling like a permissions issue, almost as if it loads and then failed permissions locks you out again but really I have no idea why its doing this.
Tested in a few places, sometimes the webconsole is also giving this.
Edit: Image removed.