And again an upgrade changed the UI. Each tab has its own close button, the one on the far right, closing the active tab is gone. If you like me liked the old behaviour better go to about:config and change browser.tabs.closeButtons to 3.
For more helpfull hints go to lifehacker.com
Update: It seems like there will be more options I’ll tweak, for example set network.dns.disableIPv6 to false, network.protocol-handler.external.mailto to true and network.protocol-handler.app.mailto to icedove