Irritating behavior – trying to open the Dropbox folder from the tray icon lists the files in the browser rather than file manager.
This answer worked for me on Lubuntu 12.04.
Install libfile-mimeinfo-perl from the repository
Run (in terminal) mimeopen -d ~/Desktop (any folder can be used)
mimeopen should prompt you to pick something to open it with. For me, PCManFM was the first option. Select the option that lists PCManFM.
Open Dropbox from the system tray. It should now open in PCManFM.