Redmine

This interface is rendered by your webbrowser (where available) and thus usually follows the interface conventions of your operating system.

In the case of Firefox, the browser seems to use settings based on the locale of your operating system instead of specifically using the start-of-the-week property. In their release announcement for their native date picker they wrote:

The calendar picker for <input type=date> is also formatted based on your browser language. Hence, the first day of the week can start on Monday or Sunday, depending on your browser language. Note that this is not configurable.

You should update your users setting in your operating system to use a different locale than e.g. English (US) but instead one where the week "naturally" starts on a Monday. In any case, this might be something that Firefox can improve. We from Redmine have unfortunately no further possibility to change this behavior of the browser.