Rated 4 out of 5 stars
I am using Thunderbird 3.1.1 and Lightning 1.0b2
I modified the install.rtf to change the maxVersions variable, as Jeff3832 did.
The extension is working but the automatic export on program termination does not seem to work.
There is a possibility as workaround:
The extension's settings allow to define a regular timer-based backup/export of your calendar.
Additionally I recommend to add the "export" and "backup" buttons to your E-mail clients toolbar, if you want to initiate a manual calendar export/backup.