Never thought about doing this before, but if you have the IE Tab extension for Firefox you can set it to always open "file:///*" in an IE tab and create bookmarks to your folders. All you have to do is copy the folder location into the address bar with a "file:///" prefix, replace all the "\" with a "/", and replace spaces with %20. Locationbar is also a great extension to go with this because you can then click on the root folders in the address bar (like Windows Vista) to go back.