I switched from Unity to XFCE by installing Xubuntu. The indicator plugin area from the Panel in Unity also appears in the XFCE Panel. Whenever I opened Firefox, I noticed that its entire menu bar would appear in the middle of this indicator area!
The easy solution is to remove the indicator plugin area from the XFCE Panel. However, I have some Unity indicators that I need to be displayed in the XFCE Panel, like the Dropbox client for example.
A better solution is to remove the Application Menu indicator itself. This is what is used by Unity to display the menu bar of an application in the Unity Panel.
I do not need this on XFCE since it shows the menu bar in the application itself. Removing this indicator is easy:
$ sudo apt remove indicator-appmenu
Restart Ubuntu to get XFCE Panel in the correct state.
Tried with: XFCE 4.12 and Ubuntu 15.04