why firefox portable create a empty folder in appdata/local ?

I am using Win 7 and I use portable firefox, but there is something strange.

when I open firefox portable, it is create folder in Local

c:/ user/ / appdata/ local/ mozila/ filefox , which is a "empty folder".

and when I leave the firefox portable, the folder is still there.

Why firefox creata a empty folder in the "appdata" ?? (　when I delete, next time i open firefox portable it will come out again. )

Is this something bug?

please help me , thanks