I had the same issue. I did a file compare and found some differences in the /wp-includes/js/tinymce/tiny_mce_config.php
I made the following changes to the 2.6 file based on difference I saw in the 2.5.1 code. My site isn’t ssl so I not sure why this made a difference, or if it was something in cache that was flushed out… I upgraded some local instances of WordPress from 2.5.1 to 2.6 and didn’t have the issue.
2.6 Code lines 53-59:
// Set up init variables
$baseurl = includes_url('js/tinymce');
$mce_css = $baseurl . '/wordpress.css';
$mce_css = apply_filters('mce_css', $mce_css);
$mce_locale = ( '' == get_locale() ) ? 'en' : strtolower( substr(get_locale(), 0, 2) ); // only ISO 639-1
New Code:
// Set up init variables
$baseurl = includes_url('js/tinymce');
//Added based on 2.5.1 file
if ( is_ssl() ) $baseurl = str_replace('https://', 'https://', $baseurl);
$mce_css = $baseurl . '/wordpress.css';
$mce_css = apply_filters('mce_css', $mce_css);
//Added based on 2.5.1 file
if ( is_ssl() ) $mce_css = str_replace('https://', 'https://', $mce_css);
$mce_locale = ( '' == get_locale() ) ? 'en' : strtolower( substr(get_locale(), 0, 2) ); // only ISO 639-1
This also fixed an issue I was having with the link editor in IE.
You may need to flush the js cache on your server, I have a plugin that adds and removes buttons in TinyMCE that does that for me…
Gregory