Unfortunately, there are a couple of slightly different issues regarding local viewing so I think if the developers were to address the problem, they'd probably want to change the code to deal with the individual issues rather just push a new release with updated labels (although I agree that this would be a step in the right direction).

Being that I'm not the author of Juicebox and cannot make any changes myself, I think the best I can do is probably add a link to this forum thread from the FAQ that is already linked to from the Juicebox error message.
I've asked the developers if this would be OK. It might at least help users find a solution to the problem.

I'm sorry that it took you so long to figure out the solution to the problem but I'm glad that you found this thread.

I've now stickied this topic so that it at least appears at the top of the 'Tricks and Tips' forum.

That would, indeed, be the ultimate solution but, unfortunately, development is rather slow at the moment.

I'll contact the developers and see if there is any other way to increase relevant traffic to this thread.

Thanks.

Maybe even change the Juicebox Builder Pro program where it says "view in browser" to put some kind of warning in. 

I realize that this Forum posting is a year and a half old, but that just means that a large majority of users are either not going to be able to use the preview or will have to tweak their settings.

If you receive the "Config file not found." message whilst viewing a gallery locally (from your computer's hard drive rather than from a web server) in Firefox or Safari, then please read on.

Recently, both Firefox and Safari have introduced security restrictions which prevent the loading of local files, resulting in the inability to preview Juicebox galleries from your computer's hard drive.
Because these security restrictions were introduced after the current version of Juicebox was released, Juicebox is unaware of the problem and displays the "Config file not found." message (instead of a more descriptive message).

There are, however, workarounds for the problems in these browsers:

Firefox (68 to 94):

• Enter about:config in your Firefox browser's address bar.

• Click past the warning via the "I accept the risk!" button.

• Enter privacy.file_unique_origin in the search bar.

• Double-click the Preference Name to change the Value from 'true' to 'false'.

Firefox (95 or later):

• Enter about:config in your Firefox browser's address bar.

• Click past the warning via the "Accept the Risk and Continue" button.

• Enter security.fileuri.strict_origin_policy in the search bar.

• Single click the Toggle button or double-click the Preference Name or Value to change the Value from 'true' to 'false'.

Safari (11 to 15):

• Pull down the 'Safari' menu and choose 'Preferences'.

• Click on the 'Advanced' tab.

• Check the box next to 'Show Develop menu in menu bar'.

• Close 'Preferences'. The 'Develop' menu will now be visible between Bookmarks and Window menus.

• Pull down the 'Develop' menu and choose 'Disable Local File Restrictions'.

Safari (16 or later)

• Go to 'Safari -> Settings'.

• Click the 'Advanced' tab.

• Select the 'Show features for web developers' checkbox.

• Go to the 'Developer' tab.

• Select the 'Security: Disable local file restrictions' checkbox.

Screenshots for Safari 16 or later can be found in this forum thread.

Please note that relaxing browser security restrictions should be done at your own risk (but it should be no more of a risk than using the previous versions of the browsers before the security restrictions were introduced).

Incidentally, it is not possible to preview galleries locally in browsers which use the Chromium web engine (Chrome, Edge and Opera). Please use either Firefox or Safari (using the tweak above) to preview galleries locally.

Also, it is important to note that this problem applies only to previewing galleries locally (from your computer's hard drive).
There is no problem once the galleries have been uploaded to a web server where they can be viewed without issue in any modern browser.

An alternative would be to set up a local web server environment on your computer using a program such as MAMP or XAMPP.
This is not as simple as just changing a browser setting but it would allow you to view a gallery locally in any browser you like (without the need to change any browser settings).

I've written short instructions for using XAMPP here.