RTE not loaded if it's not inside the TYPO3 backend frameset
Currently the RTE will only be loaded if the edit form frame is loaded as a "subframe" of the TYPO3 backend main frame. Unfortunatly this performance feature causes the bug that the RTE is not loaded if the edit form is directly opened inside a new tab/window. As this is a nice possibility to give users the opportunity to directly edit records from within another backend module I would consider the single frame loading feature buggy.
An example implementation of such a behaviour can be found in EXT:df_tools. Just install the ext, open the link checker function and execute the initial synchronisation step. Afterwards you can open the records list of each url and directly edit the related record inside a new tab. Nice TYPO3 feature, but buggy with tinymce_rte.