Correctly disable unsupported options on Firefox 27.0a1
Don't disable all options, if checked "useToolkitUI":
some tweaks also works in Library - Downloads
Make disabled options semi-transparent
Always close download panel before open any tab/window
Unfortunately we should do this before move focus to already opened library window
(part 4 for #10)
Add GUI for extensions.downloadPanelTweaker.override* preferences
(part 5 for #10)