After SPIEGEL Online has stopped its "Sidebar for firefox" the only possibility to have a quick survey about actual headlines in firefox sidebar is its RSS feed with any reader.
But in this manner it is not possible to view the headlines grouped by ressort in eye-friendly form ;-)
That's why I tried to make a style sheet which brings http://www.spiegel.de/schlagzeilen/ressort/index.htmlin a sidebar applicable form.
With this style you are able to open the page in the sidebar using a bookmark tagged with "Open in sidebar" nearly looking like the good old SPON-Sidebar.