Note: This isn't my solution, all credit goes to "nickbeee" who posted a couple of lines on the firefox site. I've had to document this for our internal users and figured that other people may benefit from a more detailed explanation. Also: I'm a stickler for long explanations with pictures.... It seems that the new security features in Mozilla Firefox 4 prevent Lotus Notes/Domino Webmail from working. Here's how to get that functionality back. First: The problem that you see looks like this; It states on the webmail screen simply (and cryptically) A problem has occurred which may have caused the current operation to fail. The problem seems to be that firefox has security-blocked your webmail. In order to unblock it, you need the Remote XUL Manager extension. To get it, browse to: https://addons.mozilla.org/en-us/firefox/addon/remote-xul-manager/ and click Add to Firefox. You'll be prompted to install the extension and then you'll be told to restart...