Does open dialog show up or not?

It might be blocked by popup blocker, I had this same issue all of sudden. So I added this instruction in description. For some reason open file dialog is treated as a popup. Can you try this?
Try to right click, holding ctrl key, anywhere on tree tabs sidebar, click on "view page info" in default firefox menu. In a little window that should show up click on "Permissions" on top. Uncheck "Use Default" under Open Pop-up Windows. And set to Allow.

EDIT:
OK! Good I'm always here to help :)