Ubiquity NotificationHere's a screen shot:
An exception occurred while running (). TypeError: window.content is null
When I type a URL in the new tab's URL text box, the web page open's in the previous tab. When I Crtl-click to open a URL in a new tab, the new web page loads in the current tab.
My best fix is to completely uninstalled Firefox, delete my Firefox settings (folder C:\Documents and Settings\my-user-name\Local Settings\Application Data\Mozilla\Firefox), and reinstall Firefox. After that, everything is back to normal.