I have no problems with firefox 2.0.0.6 and our homepage. Even when I'm logged out or logged in, doesn't matter. Could you remove the cookies in firefox for *.llblgen.com please? (only these are necessary) ?
(edit)According to the eventlog, the error was a validation error: in the form post (I guess on the login form) a '<' or '>' was received and asp.net validation stepped in. Odd...
(edit) I looked at the wrong error, this is the one at 22:23 local time (you posted here around 22:30, so I assume this is your error)
at System.String.InternalSubStringWithChecks(Int32 startIndex, Int32 length, Boolean fAlwaysCopy)
at SD.LLBLGenSiteLogic.GUI.Global.Session_Start(Object sender, EventArgs e)
at System.Web.SessionState.SessionStateModule.RaiseOnStart(EventArgs e)
at System.Web.SessionState.SessionStateModule.CompleteAcquireState()
at System.Web.SessionState.SessionStateModule.BeginAcquireState(Object source, EventArgs e, AsyncCallback cb, Object extraData)
at System.Web.HttpApplication.AsyncEventExecutionStep.System.Web.HttpApplication.IExecutionStep.Execute()
at System.Web.HttpApplication.ExecuteStep(IExecutionStep step, Boolean& completedSynchronously)
and the error: Exception type: ArgumentOutOfRangeException
Exception message: startIndex cannot be larger than length of string.
Parameter name: startIndex
Very very odd... will look into it.
(edit) found it. Very odd though, as it's a cookie date error which apparently has a wrong format all of a sudden.
We'll fix this in the build we'll be pushing online in a couple of days. Thanks for reporting this!