This bug has been reported on Ubuntu Launchpad as:
  https://bugs.launchpad.net/bugs/1359175

After restoring the previous XFCE session with a Thunar window open, icons in the address bar will lose their theme and resort to the default icon theme.

Steps to recreate:
1. Open any folder in Thunar.
2. Restart the computer, making sure to enable "Save session for future logins".
3. Log in after the computer has restarted.

Expected result:
The Thunar window looks the exact same as before the system restarted.

Actual result:
The icon for the folder in the address bar has reverted to the default theme (usually, this theme is GNOME).