for Firefox Portable and Thunderbird Portable
The Calendar extension has been dropped in favor of the new Lightning Extension which provides enhanced functionality.
As an alternative to Sunbird Portable, you can also use the Calendar Extension from Mozilla. It has the same functionality as Sunbird (scheduling and tasks) except that it installs as an extension, taking up far less space than the standalone Sunbird Portable does.
Download and Installation
You will need to right-click and then save the XPI to your computer before installing. To install within Firefox, click File - Open and then open the file. To install within Thunderbird, click Tools - Extensions - Install and then open the file. I recommend using the Stable build (based on Sunbid 0.2) but the more adventurous types can use the Experimental build.