I had navigated to a folder on my hard drive in Mozilla because it contained a bunch of documentation in HTML format. I thought I'd need to go back so I dragged the little "URL handle thing" from the Location bar to my Personal Toolbar...

...and a folder icon appeared on my toolbar instead of an "URL handle thing". To my surprise and delight, when I clicked on the folder, a menu dropped down with the contents of the folder.

Very handy feature, I hope you can use it too!