This is cause when you click on RTL (Right to Left) align in the menu on the website. It is only isolated to Firefox because you haven’t click that link in IE, so it hasn’t saved your preferences.
It is under the “RTL Language menu”. Then click “LTR languages”.
Atlernatively, this should work: http://www.yourwebsite.com/?direction=ltr (not sure if it will work as it depends on your folder structure, also substitute for your website url).
This is all assuming that you have installed the Quickstart, and have not yet removed that menu item.
I also had this issue, because I didn’t realise I had clicked on the link on the menu. But, once I clicked it back to the correct orientation everything has been fine.