Cannot Start Download By Single Click Anymore

Hi, It was OK for me to download file from a link before, but since a while ago, when I single click on any link (ex., Firefox Installer) it showed nothing and nothing happened (Normally I think dialog about if open or save/when to save should pop up). I also try other way to open the link then: 1. If I try to open a new tab on link, then a empty tab was opened (as attached image). 2. If I right click the link and click "Save Link As..." then it will pop up the save folder selection dialog. I hope to have any suggestion to fix this issue. Thanks in advance for any support.