If you type a text into the circular mail text field of OGame, the following error is shown in JS console for each letter you type:

Zeitstempel: 22.08.2013 17:34:22
Fehler: TypeError: b[(b.length - 1)] is undefined
Quelldatei: resource://gre/modules/XPIProvider.jsm
-> file:///C:/Users/Jan/AppData/Roaming/Mozilla/Firefox/Profiles/sqg13i76.default/extensions/antigameorigin@antigame.de/bootstrap.js
-> file:///C:/Users/Jan/AppData/Roaming/Mozilla/Firefox/Profiles/sqg13i76.default/extensions/antigameorigin@antigame.de/chrome/content/js/background_firefox.js
-> resource://antigameorigin.antigame.de/chrome/content/js/init.js Zeile: 20

Tested with 5.6.1 and 5.6.1.44 using Firefox 23.0.1 (error message is copied from the Beta)

Fixed in 5.6.2.1 - thanks for reporting :)