Hi Silvia,
It looks like a rule in your theme’s stylesheet is causing this issue (and for various reasons it only manifests itself in Firefox). Can you try adding the following CSS rule and see if it helps?
.widget.tribe_mini_calendar_widget table { float: none }
Some themes provide a setting to let you add custom CSS so you might try adding the rule there, if it’s available, or else simply place the above line of code in a new file within your theme, locating it as per the following example:
wp-content/themes/YOUR_THEME/tribe-events/pro/widget-calendar.css
(More information on this process can be found in our Themer’s Guide.)
I hope that helps!