Thank you for the update.
Looking back over your posts, I have now been able to replicate the problem myself.
The issue seems to be with using ../ in your baseUrl.
Firefox does not seem to like loading galleries locally (even after setting privacy.file_unique_origin to false) if you use ../ in your baseUrl to go back a level in your directory structure.
You can still use a relative baseUrl and view your galleries locally in Firefox but you would need to change your directory structure to do so, for example:
baseUrl="galleries/gallery001/",
The problem seems to specifically be with the use of ../ in the baseUrl. There seems to be no problem with using ../ in the path to the 'juicebox.js' file.
Oddly enough, you can view galleries which use ../ in the baseUrl locally in Edge (Windows) and most likely also in Safari (Mac) as long as you select 'Disable Local File Restrictions' from the 'Develop' menu first (details in this forum post).
I have notified the developers of this but there's probably very little that can be done from within Juicebox to affect core browser functionality.
At least, knowing the problem, you should be able to work around it (by changing your directory structure and ensuring that you do not need to go backwards to reach the gallery folder via the baseUrl).
I hope this helps.