What's the point of disabling such button? The user can achieve exactly the same functionality with keyboard, context menus, bookmarks, mouse gestures... If your program breaks when the user makes use of his browser's history, you should fix whatever breaks in your code rather than annoying the user by crippling his browser.
Update
Alright, you've made it clear that you want an answer to your question, not a solution to your problem. Sorry for wasting your time.
The answer is that you cannot disable the browser's back button from the server. The proof is that, if such thing was possible, webmasters from porn, scam and spam sites would do it all the times it so you would not be able to leave their sites. Your only chance is to convince your Firefox users one by one to go to the View-> Toolbars-> Customize
menu and drag the navigation buttons out of the bar. Alternatively, you can try to make them download and install some sort of malware that does this for them.