Unfortunately, very recently, Firefox 68.0 (released on 9 July 2019) introduced a new security restriction (very similar to the Safari 11 issue) which blocks the loading of Juicebox galleries locally (from your computer's hard drive). (As you have discovered, there is no problem once the gallery has been uploaded to a web server.)
Here is the relevant entry from the Firefox 68.0 Release Notes: https://www.mozilla.org/en-US/firefox/6 … easenotes/
Local files can no longer access other files in the same directory.
It's an unfortunate turn of events but, thankfully, not an insurmountable problem as there is a workaround.
Here are the steps required to relax this security restriction:
(1) Enter about:config in your Firefox browser's address bar.
(2) Click past the warning via the "I accept the risk!" button.
(3) Enter privacy.file_unique_origin in the search bar.
(4) Double-click the Preference Name to change the Value from 'true' to 'false'.
***Please do so at your own risk*** (although this should be no more of a risk than running the previous version of Firefox (67.0.4)).
This should allow you to, once again, view your galleries locally in Firefox.
I hope this helps.