Itâ€™s strange that you can accomplish this in IE while you canâ€™t make it happen in Firefox. Well, thatâ€™s because the development team has decided that itâ€™s potentially annoying and harmful for web developers to change the text of Firefox status bar whatever way they want.
So the simple answer is, you canâ€™t. But thereâ€™s always some sort of switch that would turn on or off the custom modification of the status bar text.
Simply type about:config in the Firefox address bar and it would take you to a vault of hidden advanced settings not meant for common users. Filter: disable_window_status_change and you will see that Firefox has come disabled of window status change by default.