Keyconfig extension
Description of Keyconfig
The extension Keyconfig allows you to change keyboard shortcuts. [Firefox, Thunderbird, SeaMonkey (maybe)]
Shortcuts defined via a key 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). That means you can change the shortcut for "Decrement URL" (Magpie) but not for "Undo Close Tab" (Tabbrowser Extensions).
Custom Keys
You can add the following lines to the file prefs.js in your profile to get the described results. All instances of Firefox/Thunderbird must be closed before editing this file. After editing start FF/TB and open the Keyconfig window to add your keyboard shortcut.
You could also add the lines to you user.js, but after starting FF/TB you have to delete the lines from the user.js. If you won't delete them, they will be applied again with the next start of FF/TB and will overwrite (delete) the assigned keyboard shortcuts.
Template
user_pref("keyconfig.main.xxx_key__NAME", "!][][][FUNCTION");
