Ja, hatte erst vor kurzem das selbe Problem, mit Mozilla ist es gegangen, IE und NS sind ausgestiegen.
Da haben in der php.ini einige Zeilen gefehlt.
So sollte es klappen
------------php.ini---------------------------------------
session.save_handler = files ; handler used to store/retrieve data
session.save_path = /tmp ; argument passed to save_handler
; in the case of files, this is the
; path where data files are stored
session.use_cookies = 1 ; whether to use cookies
session.name = PHPSESSID
; name of the session
; is used as cookie name
session.auto_start = 0 ; initialize session on request startup
session.cookie_lifetime = 0 ; lifetime in seconds of cookie
; or if 0, until browser is restarted
session.cookie_path = / ; the path the cookie is valid for
session.cookie_domain = ; the domain the cookie is valid for
session.serialize_handler = php ; handler used to serialize data
; php is the standard serializer of PHP
session.gc_probability = 1 ; percentual probability that the
; 'garbage collection' process is started
; on every session initialization
session.gc_maxlifetime = 1440 ; after this number of seconds, stored
; data will be seen as 'garbage' and
; cleaned up by the gc process
session.referer_check = ; check HTTP Referer to invalidate
; externally stored URLs containing ids
session.entropy_length = 0 ; how many bytes to read from the file
session.entropy_file = ; specified here to create the session id
; session.entropy_length = 16
; session.entropy_file = /dev/urandom
session.cache_limiter = nocache ; set to {nocache,private,public} to
; determine HTTP caching aspects
session.cache_expire = 180 ; document expires after n minutes
session.use_trans_sid = 1 ; use transient sid support if enabled
; by compiling with --enable-trans-sid
----------------------------------------------------------------------
Sloter