Your custom dimensions match well with the Small, Medium and Large image dimensions that Juicebox-Pro uses by default for its Multi-Size images (800 x 600, 1024 x 768 and 2048 x 1536 respectively).
These default dimensions have not changed since Juicebox-Pro was introduced but I think they are still pretty good default values that cover most scenarios.

Of course, one person's site might have a completely different target audience to someone else's site whereby visitors might be more inclined to view the site on a mobile device (or conversely, on a large desktop monitor), in which case, you might want to change these values accordingly.

If you are using Multi-Size images for a Juicebox-Pro gallery within Showkase, there's no quick and easy way to tell which image size is being served up without delving into the browser's developer tools to see if the image originates from the 'images/' folder (Medium), the 'images/small/' folder (Small) or the 'images/large/' folder (Large).

I've just checked and Juicebox v1.5.1.2 galleries still display and function OK in Firefox 146.0 (64-bit) on my WIndows 11 PC.

If you are having trouble viewing galleries locally in Firefox (before you upload them to your web server), then please try changing the setting noted in this FAQ: https://www.juicebox.net/support/faq/#local

If you continue to experience difficulties, please let me know more about the problem you are facing.
Let me know whether the problem happens when viewing your galleries locally or from a web server.
Also let me know exactly what the problem is (what happens or does not happen) and if you see any error messages along the way.
With a little more information, I should hopefully be able to help further. Thank you.

Google Translation:

I don't see a speed increase when trying this on a Windows PC and, knowing how JuiceboxBuilder works, I don't actually understand why there should be a speed increase on any system but I'm genuinely glad it works for you!

I'm not 100% sure of your exact method.

If I do this (manually replace the images in the 'images' folder and click 'Save' on the 'Publish' panel), JuiceboxBuilder immediately reinstates the original images (as I've bypassed the 'Add Images' routine on the 'Images' panel and JuiceboBuilder has no knowledge of the new images in the 'images' folder).

Are you manually adjusting the 'imageURL' entries in the gallery's 'config.xml' file (to reflect the new image filenames)?
How are the thumbnail being generated?

Maybe I'm not following your instructions correctly.

If you don't need JuiceboxBuilder to resize your images, then you could deselect the 'Resize Images' checkbox on the 'Images' panel. This should speed up the 'Add Images' process a little.

Where did you get that information? The source might give me a clue as to what is going on. Thanks!

As time goes by, I think it is increasingly unlikely that an update will be forthcoming.

I'm still here as official support but I'm neither the author of Juicebox nor the person making the decisions.
As such, I've not heard any official word on the future of Juicebox.
Having said that, I've been pushing for an update behind the scenes for a while now (with suggestions and code) but I do not know if anything will come of it.

I'm sorry it's not better (or more certain) news.
In light of this, I'd understand if you wanted to migrate to a different web gallery program.
Thank you for your continued support over the years.

No problem! I hope you're able to resolve your problem.
If you still have trouble with it, just post back here and I'll try to help further.

It certainly sounds like Norton might be quarantining your zip file.

I have what sounds like a similar setup (but without Norton): Windows 11 Version 24H2 (OS Build 26100.6484) with Windows Defender up-to-date and Lightroom 4.

I can click/double-click/extract the Juicebox-Pro zip file just fine on my own PC.
I use 7-Zip as my zip manager of choice but this should not matter. As soon as the file is accessed (by whatever means/program), Norton will kick in and scan the file (and act accordingly).

In any case, I just tried extracting the Juicebox-Pro zip file using Windows' own native zip functionality and it worked fine.

Have you tried checking your Norton logs to see what has recently happened?

Also, I'd double-check that Norton is actually fully suspended as a zip file disappearing when clicking on it is not normal behavior and sounds very much like a third-party is interfering.

If you need another link to re-download the Juicebox-Pro zip file, please fill in the online Download Link Request Form and I'll send you a new one. (I would have already done this but I cannot find the email address that you use here in the forum in our transaction records.)

Incidentally, I should reassure you that, as long as you downloaded the Juicebox-Pro zip file from your original download link, the file does not contain any malware at all so if Norton is quarantining the file, its a false-positive. If this is the case and you can find any details relating to this, please forward them to me at steven@juicebox.net and I'll file a false-positive report over at Norton so that they can clear Juicebox's reputation in their next database update. They'll likely want to know the Norton module flagging the issue (e.g. real-time scan, manual scan, ransomware check), the version of Norton you are using, the database version and the description of the false-positive finding (i.e. what virus it thinks it found).

I'm glad you're making good progress.

Here are some further notes which may (or may not help)!

If you don't want the Splash Page, then you can turn it off (without changing the dimensions of the gallery or anything else on the web page) by setting showSplashPage="NEVER". It sounds like you might actually want the Splash Page (and the gallery to be expanded to fill the page), though.

You could always use the Back Button to achieve this

The space between the thumbnails and the main images will depend on a number of factors such as the number of thumbnail columns, the width of the thumbnails, the shape of the main images (e.g. landscape vs portrait), the width of the browser window/monitor. Juicebox does its best to find a good compromise in most scenarios.

You can force a certain Screen Mode by setting either screenMode="SMALL" or screenMode="LARGE" (but bear in mind that this will apply for both desktop browsers and mobile devices).

Gallery layout is a very personal things and it's really more about what you like yourself. I'd only recommend something if I think there's a problem.
New 1 is so close (in terms of gallery dimensions) to not needing a Splash Page. It just boils down to whether or not you actually like the Splash Page and want to use it.

[Posts split into new thread to separate from original 2023 thread.]

@fabienismyfriend

Your galleries seem to be working as expected when I view them on mobile devices.
If a mobile device has been detected and the gallery is embedded on a web page with dimensions of less than 100% x 100% or if there is other content on the page (i.e. if the gallery is not the only element on the page and does not take up the full page), then the Splash Page will be displayed.
The Splash Page is a placeholder for the gallery which, when tapped, will expand the gallery on a new page of its own (to give the gallery more space).
By default, the Splash Page displays the first image in the gallery. You can change this to any image you like using the splashImageUrl configuration option (in JuiceboxBuilder-Pro's 'Customize -> Splash Page' section).

There are a few things you could try.

(1) If you simply do not want the Splash Page to appear, then you can set showSplashPage="NEVER" ('Customize -> Splash Page').
(2) Additionally, if you want the Large Screen Mode version of your gallery to be displayed on all devices, then you can set screenMode="LARGE" ('Customize -> General') but please bear in mind that some gallery elements may appear too small on mobile devices.

You could avoid the problem by ensuring that your gallery is the only element on your web page and that its dimensions are 100% by 100%.
Currently, your gallery has a height of 92% and there is a table on the page alongside the gallery.
If you were to remove the table and change the height of the gallery to 100%, then the gallery would fill the page and the Splash Page would not appear (but the gallery would still be displayed in Small Screen Mode on mobile devices which would be beneficial when viewing on small screens).

I'd recommend using the 'index.html' page from a gallery built with JuiceboxBuilder-Pro as a template (where the gallery is the only element on the page).

Also, if you check your current gallery's embedding page with the W3C Markup Validation Service, you'll see that there are some HTML errors on the page (such as a missing </body> tag at the end of the file). The info entries do not need attention but the errors really ought to be fixed.
If a web page does not contain valid HTML code, then the web page might be rendered by the browser in an unpredictable manner. Some browsers are more tolerant towards errors than others but if the code on the page is valid (i.e. no errors), then you can be sure that the page will display with consistency across all browsers.

I hope these notes point you in the right direction.

Perhaps the two most influential factors in the loading of a gallery are the speed of the web server (where the gallery files are hosted) and the user's internet connection. Unfortunately, there is little you can do about either of those.

You could maybe tweak the size of your images. From memory, if you allow JuiceboBuilder-Pro to resize your images at default settings (medium sized images at 1024 x 768), the resulting images tend to be around 120KB each. This is significantly smaller than your current average of 300KB and images should, therefore, load faster (and they should still hopefully be of a reasonable quality for web gallery use).
You could also take advantage of Juicebox-Pro's Multi-Size Image Support (see here for details) where the gallery stores different sizes of images and serves up smaller ones on mobile devices. It might make a difference (depending on factors such as the device's screen size and pixel density).

I don't think the imageScale mode will make a big difference to the loading speed but the imagePreloading value (Large Screen Mode only) might. (Both of these configuration options can be found in the Main Image Options section here). When setting the preloading to the default value of PAGE, Juicebox will preload all images on the current thumbnail page so reducing the number of thumbnails per page should speed up the initial load a bit. Alternatively, you could set the preloading to NEXT, whereby only the next image in the gallery is preloaded. This should provide the fastest possible loading time (less initial burden on the browser) but I can't say whether it will make a difference on your Android device. It's something you can experiment with, though.