<af:commandtoolbarbutton accesskey="o" action="OpenDetails" text="Open" />
The accessKey must be a single character and in firefox, in order to use the access key you have to hit Alt+Shift+o.
What if you want to use an accelerator key for other types of keystrokes? Menu Items are the only component that supports accelerator keys so for example to add a hot key for CTRL+0 you would need to add a menu item somewhere on the page
<af:commandMenuItem action="OpenDetails" accelerator="ctrl 0" />
A menu item needs to hang inside a af:menu and af:menu needs to hang inside a menuBar so if you only want to use the accelerator feature without displaying a menu you will need to hide the menu container (menuBar).
using inlineStyle="display:none". Now have the menu item perform the same action as the button and you have just created an accelerator key stroke for your button.
Oracle Documentation on Accelerator Keys