As per bug 718015 comment 15: The text change doesn't take effect until the next time the widget is reloaded (via new window or browser startup). It does seem a little bit inconvenient if someone wants to test a specific persona but it was the simplest implementation at the time. Alex pointed out to me that you can register a listener on the following observer service notification: http://mxr.mozilla.org/mozilla-central/source/toolkit/mozapps/extensions/LightweightThemeManager.jsm#275 Further we should also check how we react on OS wide font changes and if it also would need a listener for a notification.

When Australis lands widget will be deprecated so we won't be working on this.