I have viewed your 'HMM Aerial Photography' gallery on my iPod Touch 6 running iOS 11.2.2 in Mobile Safari and Mobile Firefox 10.4 and the results look very similar in both browsers.

If you are referring to the browser address bar in horizontal orientation, then I think the difference you are seeing may simply be due to the different design choices that the browser developers have made. Apple (Safari) chooses to hide both the address bar and the navigation bar whereas Mozilla (Firefox) chooses to hide the navigation bar but not the address bar.

Also, please bear in mind that Juicebox is not tested on Mobile Firefox and so mobile Firefox is not an officially supported browser (although viewing galleries in Mobile Firefox should not normally cause any problems).
We support (and test in) the browsers which come bundled with mobile devices (Mobile Safari on iOS and the stock browser on Android) and also Mobile Chrome (both platforms).
Mobile Chrome is more mature (and thus more stable and predictable in its behavior) than Mobile Firefox currently is.

If you let me know some more about your problem, I might be able to help further.
Please let me know what iPhone you have, what version of iOS you are running and what version of Mobile Firefox you have installed.
Please also let me know what gallery you are referring to so that I can try to replicate the issue you are describing and see the problem for myself.
I cannot be sure that I'll be able to help further (what you are seeing might just be down to the different ways that different browsers work) but I'm certainly happy to help further if I can.