For those early adopters interested in installing Clippings 3.0 milestone releases (numbered 2.99.x) on a non-English Firefox installation, please note that these releases do not yet have any of the new UI strings localized. This is the cause of the error messages reportedly appearing below the Firefox or Thunderbird application windows.
This problem has been documented in bug 18572 which will be fixed before the Clippings 3.0 release candidate. In the meantime, the solutions that I can suggest are to either stick with Clippings 2.6, or wait until the release candidate is released.
(With thanks to the reporter of bug 18572 for the heads up.)