I am not sure how long this has been around in Firefox, but at least in the latest version of Firefox 3 beta for Mac I stumbled cross this trick. This morning while working on other things I accidentally dropped a folder from the Finder into a Firefox window I had in the background. Imagine my surprise when I saw what you see in the image above. Firefox displayed the contents of the folder in a file browser like structure. I could navigate up and down the directory tree and there was even an option to turn on and off viewing of invisible files. I did notice that it won’t allow you to navigate to the root of your hard drive, but files that can be viewed normally in a browser (like image files or Quicktime movies) will show up right inside Firefox. They have also done a very nice job with the user interface. It sorta reminds me of .Mac.
Now normally this shouldn’t be surprising considering that many browsers can view file systems on remote servers (if the feature is turned on and allowed in the server configuration), but this works locally. I also know that Internet Explorer on Windows has been able to do this trick for years, but this is on a Mac. The trick also does not work in Safari, at least not in the current version of Safari running under OS 10.5.2.