2.5.1. cant use html eidtor

after i upgraded to 2.5.1., i cannot write a post in html way, but just visual way. the button of html doesn’t work!

Firefox shows the error in: editor.js (line 82)

‘tinyMCE is not defined
go(“content”)editor.js (line 82)
onclick(click clientX=0, clientY=0)post-new.php (line 1)
[Break on this error] var ed = tinyMCE.get(id);”

can someone help me? It’s emergent! I have to write a post today.:(
thank you!