@hnsaifi - On the iPhone, after you click "Choose Images", can you scroll all the way to the top to see the Edit Gallery option? I'm seeing that in my testing, which if it is what you're seeing, is a different issue than the Firefox problem.
I haven't been able to duplicate the Firefox issue yet. Since it is a relatively new feature, have you cleared your cache lately? Also, can you create a gallery the typical way (via Add Media in a post)? They're both using the same modal action, so if one works and not the others, it can help diagnose where the failure is occurring.
Thanks again for your patience and help.