I encountered this error after every GoAgent update and finally this solved my problem.
One of the thing I hate about firefox is that when you have internal certificates like those issued by HP Integrated Lights-Out’s (iLO) web page, you have to confirm to download the cert and add it to your cert store. There is no one-click solution to this as yet and neither can I disable this in FireFox even if I only intend to use it in an intranet environment.
More troublesome is when I update the firmware in my iLO and hence a new certificate is generated and when you try to launch the web page for iLO access again you will encounter the following error.
Error code: sec_error_reused_issuer_and_serial
It took me a while to figure it out from googling, and the solution is very simple. Just delete the cert8.db from your FireFox profile, who is usually found in C:\Documents and Settings\\Application Data\Mozilla\Firefox\Profiles\. The easier way is just searching for cert8.db in your computer and delete that file.
Personal favorite add-ons. Constantly updates.
Adblock Plus: Deal with ads.
Easy Access: Helpful tools developed by Mozilla CN.
Firebug: Intercept their websites.:)
HttpFox: Craft Http request. Don’t be evil~ :)
HttpRequester: Monitoring Http traffic.
EPUBReader: Read epub files. Can also help you figure out how to parse them.
Flash Video Downloader: Download valuable videos.
Clearly: Helpful tool delivered by Evernote. The best clip tools ever used.