If you try to browse an Android device from Thunar in XFCE over Bluetooth, it will try to launch Nautilus, which of course isn't installed in XFCE.
The Arch Linux Wiki has a useful script to allow Thunar to browse files on Android over Bluetooth. The script uses obexftp, which must be installed.
obexftp must also be installed in Android. I've read that some Android devices have the service running, but mine didn't.
Bluetooth File Transfer and ES File Explorer File Manager both install the service.
To be honest, I didn't have much success browsing files. In particular, image files from the phone's camera took a long time to browse, and half way through the folder, Thunar would give the same thumbnail to every file.
But I thought I'd document what I did in case I need to come back to it, or in case somebody else finds it useful.