Have you modified the config file for Firefox? Do you have other remapping add-ons installed? Because they will interfere. Back button only works if there is a history to go back to (like back arrow) and it works every time. Some webpages block the history, nothing can be done about that. Since you are the first one who complained I assume these are the type of webpages you're visiting. Consider that, 400 people do not have any issues.