on Firefox (46.0 for PC, but I suspect the issue is wider) and a large display, using the demonstration at http://angular-ui.github.io/ui-calendar/ the div.fc-right (containing the forward and back buttons) jump between being placed to the center and the right of the calendar toolbar, depending on whether the month names are long or not - moving from July to September should replicate the problem.
on Firefox (46.0 for PC, but I suspect the issue is wider) and a large display, using the demonstration at http://angular-ui.github.io/ui-calendar/ the div.fc-right (containing the forward and back buttons) jump between being placed to the center and the right of the calendar toolbar, depending on whether the month names are long or not - moving from July to September should replicate the problem.