For instance, shortcuts like ctrl+shift+w for "close window", and so on.
You can get the "Key Config Extension" if you want to customize the key assignments: http://mozilla.dorando.at/readme.html or "Customizable Shortcuts" at: https://addons.mozilla.org/En-us/firefox/addon/customizable-shortcuts/
You can also go to about:config and add entries for "ui.key". See Customizing Mozilla for details, at http://www-archive.mozilla.org/unix/customizing.html -- the section titled "How Key Bindings Work in Mozilla" explains about XUL & XBL. It appears that the XUL bindings are stored in comm.jar and XBL bindings are in XML files like "bindings.xml".