phus/goagent — comment on commit d2f034e236c362a8119386a52c68cadb082862cd
File:  line 0
By ayanamist:

Firefox 3 will actually remember certificate serial numbers and reject a new certificate with the same serial number as a previous certificate (coming from the same authority, that is), so make sure you’re keeping track of serial numbers.
Source from: http://www.devklog.net/2008/05/25/generating-ssl-certificates-that-will-make-firefox-3-happy/