Theme displaying incorrectly (Yes, another IE/Firefox issue)

Just so we are all on the same page, I spent hours searching for answers to this or even some help that would point me in the right direction. The ‘Our Mission’ tab on my website is displaying on Internet explorer on its clicked mode when the pages initially load. On Firefox, as we probably already know, the function displays unclicked as it should. If someone can either provide me with a simple fix or at least point me in the right direction, I would appreciate it.

Thanks in advance,
Josh