Use the keyconfig extension to easily change the keyboard shortcuts in your Mozilla Firefox/SeaMonkey web browser or Mozilla Thunderbird e-mail client
Shortcuts defined via a can be changed but only changing those which itself call a function (those with a command or oncommand attribute) has an effect (all others fulfill only cosmetic purposes it seems and are grayed out).
The configuration screen can be accessed via Tools->Keyconfig... or accel+shift+F12 on all supported windows. Currently there is only one list of modified keys per application, so changing a shortcut in one window will also affect keys in other windows if they use the same key name.