TBB 3.6.6 won't import certificate

In Certificate Manager on the Servers tab I am trying to import a certificate. The GUI allows me to choose a file and the dialog closes but the certificate is not imported and no error dialog is shown.

I have tested the same certificate in vanilla Firefox ESR 24.8.1 and TBB 3.6.2 (the only old version I have) and both work fine.

Trac:
Username: ZcbCkyj5