Hello everyone.
I am currently using 2.6.1 and have also been stumped by the same visual editor problem and tried everything mentioned in these posts to no avail.
My problem turned out to be the Viper Video Quicktag plugin and disabling it wasn’t enough, I had to delete the entire directory for that plugin. Despite being an up-to-date version of the plugin it was definitely conflicting with the WP editing window.
As it turns out, the WP WYSIWYG editor is plenty useful enough anyway.
Thanks for everyone’s suggestions and good luck.