This issue was caused by a change introduced in Firefox and Thunderbird 14 which broke Clippings. As a result, Clippings is currently incompatible with Firefox and Thunderbird versions 14 and 15, and the bug fix requires a new release.
The new release has been submitted for review by Mozilla Add-ons reviewers. An announcement will be made here once it is made available for download. In the meantime, users should confirm that automatic add-on updates are enabled so that they get the new release when it is available.