Requesting Help with Horizontal Menu Going Vertical in Firefox

Hello,

I am using a Theme (Charitas) which was purchased from ThemeForest but has now being sold by WPlook. In doing so, I am getting difficulty in either updating it or finding support.

However, the problem I am facing is that since the last few weeks…sometimes when I click on a certain page…the horizontal primary menu stacks on top of each other towards the right side of the screen..and becomes vertical.

This problem is happening in Firefox. I checked in Chrome and the Menu remains intact. Please let know what can be done to fix this problem…considering I may not be able to update the theme.