Just started upgrading some themes ready for 3.0 when I came across a bizarre issue. The theme I'm currently working on doesn't have a default header image but offers the standard custom header option so that users can upload their own.
In 2.9.2, this is achieved using
define('HEADER_IMAGE', ''). Using the same approach in 3.0 causes my copy of Firefox (XP/3.6.3) to hang on the Appearance/Header page initially without fully completing the page download. Returning to that page after accessing another Admin option is impossible and Firefox has to be killed via a commandline switch.
I can replicate the issue in TwentyTen by changing:
define( 'HEADER_IMAGE', '%s/images/headers/path.jpg' );
define( 'HEADER_IMAGE', '' );
and also by simply commenting out the HEADER_IMAGE define. IE8 manages to remain stable but does display a js error on the second load of Appearance/Header.
Can anyone else replicate this behaviour?