It is realy excellent editor, but maybe there should be made some change in browser.js, because it is not very practical to sniff them this way. When you replace whole script with this three lines
var d=document;
var moz=(d.designMode&&!d.uniqueID); //op9+ and gecko
var ie=(d.uniqueID&&!d.childNodes); //ie and !ie4
someone probably discover reason why it is working in Opera just partly.
Function iframe() in wym.js seems to me better to be at start of wym.js, when I replace it (and remove overflow hidden at toolbar), it starts to work even in very old Firebird 0.7 (now Firefox).