Keydown does not always work in Firefox

Keydown does not always work in Firefox

I develop a small web app. The user can click on button and a modal dialog
appears. On this dialog, the user can choose from different options. I
used the jquery keydown callback to give the power to use the keyboard
seleceting the different options.
I bind this way:
$('body').bind('keydown',this.keydown_handler);
It works perfectly in Chrone and in Safari. But sometimes in Firefox it
just does not work. When I switch between browser tabs, it will be fine.
When I click somewhere in the dialog it works fine again. But sometimes I
click on one of the option provided, It stops working. The options are
images in different divs.