What you are seeing in Firefox is the gallery being displayed in Small Screen Mode (where thumbnails and main images are displayed on separate pages).
Small Screen Mode is usually used only on mobile devices (or when Small Screen Mode is forced by setting screenMode="SMALL").
(More information on Screen Modes can be found in the Gallery Tour here.)
If all three screenshots are from the same gallery, then screenMode must be set to AUTO, otherwise a certain Screen Mode (either Large or Small) would always be used on all devices and in all browsers, so something must be triggering Small Screen Mode to be used in your Firefox browser.
Here are a few things to check:
Check that your Firefox browser is not in 'Tools -> Web Developer -> Responsive Design Mode'.
Check that your 'View -> Zoom' is set to 100% and not zoomed in or out.
Check that you do not have any browser extension installed and active which is currently changing the User-Agent string (for example, to mimic a mobile device).
I hope that this helps or at least points you in the right direction.