To view the issue, open the story in Docs view in both Firefox and another browser.
Chrome ✅
Firefox ❌
Firefox (with the transform style disabled) ✅
In Firefox, the zoom feature uses transform styles (see 12845) - unfortunately this breaks positioning on elements that require position: fixed when in the "Docs" view.
Normally a position: fixed element's position would correspond to the window but in Firefox it corresponds to the element with the transform style. This is confirmed by disabling this style in the Firefox dev tools.
The issue is present regardless of actually using the zoom feature. I have tried to disable the zoom feature (see 16771) in the hopes that it might also remove the styling used for zoom, however, the styling remains.