Ever since Sunbird 0.3a1 was released, and even before, there has been a constant current of questions regarding the status of the Calendar extension for Firefox, Thunderbird, and Mozilla Suite/SeaMonkey. These questions have, for the most part, all had the general form “Are you going to update the Calendar extension as well?”
To begin with, throughout the development of Sunbird 0.3a1, all of the code for the Calendar extension was maintained, and changes and improvements made to Sunbird were also made to Calendar. At this point, anyone who wants to ought to be able to create an 0.3a1 equivalent version of the Calendar extension for their own use. You can follow the directions at the project page to do so.
The next logical question, of course, is “Why are there no new public versions of the Calendar extension available?” The short answer to this is ‘resources’. It should come as no surprise that the project simply does not have the same resources of say, Firefox. We have a very small set of build machines that are currently being forced to do double-duty, also building Lightning. Attempting to make these machines also build Calendar simply is not possible. It takes away too many build cycles.
The other main resource we’re lacking is manpower. Although the Calendar extension and Sunbird share a large majority of the code, there are still differences between them. Before we could even consider releasing an updated version of the Calendar extension, it would need to be thoroughly tested, on each of the 3 main platforms, in each of the 3 programs. (That’s 9 different, complex test sets.) After that, there would be the additional strain on the development team to triage bug reports that came in, to determine whether they were Calendar specific, or also applied to Sunbird (or to Sunbird and Lightning). Our development team is fairly small
so adding this additional burden would delay future releases of Sunbird and Lightning.
To conclude, development on the Calendar extension has continued alongside Sunbird development. Currently, if you don’t have the ability to build Mozilla projects on your own, Sunbird is your best option. Furthermore, it is important to note that the future of the Calendar extension really is Lightning. (Sunbird will, of course, be the future stand-alone.) In the short-term, we’re working hard to make Lightning compatible with Thunderbird 1.5. We do recognize that the current situation, having no updated Calendar extension for the 1.5 version of Thunderbird, leaves our users in a difficult position. Hopefully, this explanation has helped to elucidate why there really is no other option. Until such time as we can sort out the remaining obstacles for Lightning in TB 1.5, users who have upgraded from TB 1.0.7 are encouraged to use Sunbird.