I have noticed this problem in a few sites. For example, EFF.org is with forward secrecy in its openSSL implementation. When I load it from Tor Browser Bundle and check the "Padlock" icon on address bar, it only says "AES256, 256 bit keys". When I load the same EFF site on plain Firefox, it says "TLS_ECDHE_RSA_WITH_AES_128_GCM_SHA_256" ..
From what I found and read online, "ECDHE" part means Perfect forward secrecy. So why is Tor browser bundle is disabling it if PFS will be providing better security?