I have a problem with my forum where IE works fine though other browsers like Firefox do not.
It's when users are replying to a message and inserting formatting code like bold and italic.
With IE, it inserts the code where the cursor is or around the text selection (as you would expect). For other browsers, it puts the code at the bottom of the message no matter where the cursor is.
compare inc_code.js between the 2 versions. But there have been quite a few security related updates since 3.4.03 , so it would still be better to upgrade.