Answer given by Brock Adams in comment:
To see Greasemonkey errors, open the browser console (Ctrl - Shift - J). [...] The web console is tab-specific -- which excludes much add-on activity. The browser console catches everything that's catchable. The Firebug console catches most things still.
Plus Nelson comment:
This worked for me, but only after enabling "Show Content messages" in the gear menu of the browser console.