'''Update''': this seems to be an issue with IE Tab. I have two different versions of IE Tab. The behavior is slightly different, but in the end, if you launch a URL from an external application that is marked to use IE Tab, then you get either a blank "new tab" with the URL in the address bar, or you get just a blank "new tab" with nothing in the address bar. I have Outlook 2007 on Windows Vista, and Firefox 4.0. After the update to Firefox 4.0, when I click a link in an outlook calendar item, Firefox (if it is already running) only opens a new tab without following the link. If I click the link in Outlook again, a new Firefox window is open. My settings are for a new tab to be opened. I have tried this with and without Tab Mix plus, but it happens in either case. If I set Tab mix plus to open in a new window, or Firefox is not currently running, then it works as expected. UPDATE: there are some URLs that are more susceptible to this than others. Here's one that always seems to exhibit this behavior: https://www.rooms.hp.com/attend/default.aspx?key=xxxxx