It happens for the button "New Task" in task view and for "New Event" in the today pane on Windows with Aero theme. Steps to reproduce: set all the calendars in read only mode the mentioned buttons get disabled and the labels text get the grayed out look, but the icons don't change.
Created attachment 667335 [details] [diff] [review] patch The patch fixes. I've also tried to assign the class "toolbarbutton1 calbar-toolbarbutton-1" (the same of the buttons in the toolbar) instead of assign directly the opacity. It works but the text in New task button gets a wrong position.
Comment on attachment 667335 [details] [diff] [review] patch Good catch, I haven't know this buttons can have a disabled state. This looks good. Thanks for this patch.
Pushed to comm-central: http://hg.mozilla.org/comm-central/rev/5de93391819b