Sorry for my carelessness. I have find the option.
But It didn't work with firefox.
I want user to download file with less than 2 connections,but don't limit user to browse folder page.
So I set "max simultenous downloads from single address" :2, and speedlimit:50K
The other limit option :disable
Result:
IE and Opera: user can only download file with 2 connection,and at same time the user can browse folder.
Firefox: user can only download file with 2 connection,and at same time the user can't open folder page,just waiting till file downloaded.
I have test with 138 and 126, occurred same question.