Can’t enter in admin pannel

Well, I’ve just update to 2.6
Everything seems to be right, update was succesfull, I reactivate all my plugins but suddenly the blog said me to re-log. I thought was a matter of cookies because of the new update or something like this.
But not, I can’t enter. I tried to delete cookies and sessions but issue persists.
Starts to charge but the screen still at the same point.
An interesting thing is that I can enter in wp-admin by IE but not in FF.
I tried to disable the plugins but nothing changes.
Can someone help me?
Thanks in advance.