I don't know which add-on I installed, then Make Link start to return the wrong result. It get the "[object Object]".
I spend 2 or 3 more hours, then I found that both Make Link and Digg This! define the function: getSelectedText(). And Make Link invoke getSelectedText() in Digg This!, so it get "[object Object]".
The solution is to remove Digg This!.
Or you can pick one add-on and rename getSelectedText(), then re-package it.