Unfortunately, Firefox 68.0 (released yesterday, 9 July 2019) has introduced a new security restriction which blocks the loading of Juicebox galleries locally (from your computer's hard drive). (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.
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 it should be no more of a risk than running the previous version of Firefox (67.0.4)).
I hope this helps.