Signing Firefox extensions

My company is developing a Firefox 1.0 extension and we would like to
digitally sign the XPI.  However, Firefox will not recognize the XPI as
being signed.  We have a certificate from a reputable CA, and have attempted
to sign the file using Java's keytool/jarsigner and Netscape signtool.  In
both cases, the resulting XPI contains a META-INF directory containing the
files manifest.mf, zigbert.rsa and zigbert.sf.  ( For the Java tools the
names of the rsa and sf files are a prefix of the owner of the certificate).

In examining the problem, I also tried to install the "simple signed XPI
testcase" at http://www.mozilla.org/projects/xpinstall/signed/testcases/,
but Firefox did not recognize this as being signed either.  The only XPI
that appears in Firefox as being signed is
http://www.swiftview.com/product/sv_710/svinstall_p.xpi (cf Bugzilla post
https://bugzilla.mozilla.org/show_bug.cgi?id=273406 ).

Has anyone else experienced this problem, and know if special steps must be
taken in signing XPIs?  Also, to examine the problem further, I would like
to try to install other signed XPIs, but in general I have had difficulty
finding signed XPI files.  If anyone knows of any others please let me know.

Tom