you'll first have to click on the "I Understand the Risks" link for the "Add exception..." button to show up

Thank you so much for yout answer. Nevertheless, I don't even get that... screen! I immediately get the screen in attach. I suspect my IT folks here are tweeking FF before deploying it and they also restrict unregistered executable apps so no joy for FF Portable... :-(

ok, i didn't read the support article thoroughly enough - it even states that this dialogue cannot be surpassed by adding an exeption. you're very probably right that this problem arises because the same cert is used for different sites. this support article describes a work-around but it's likely not applicable to your situation, since you'd also have to delete the certificate authority & that would create all sorts of problem since your production site will likely use a certificate from a valid authority.

please contact your IT department to either fix the certificate issue on the two different servers or to enable you to run a portable firefox version where you can mess with the certificate store and the trusted authorities in order to access the test server.

Thanks again mate! I did an export of the issuing Certification Authority of our production site certificate and the internal site started showing the page you had send the screen-shot in your first answer and I could add exceptions... :-) Curiously enough there was also no impact at all on the production external site and it just cruises through... :-)

Problem solved... for now at least... :-) I'll keep those exports safe though.

Thanks again!

Forgot to say that after the export of the certification authority I deleted it from the list...

I have just installed a firmware upgrade on my HP network attached printer, and am now no longer able to get to the device at https://192.168.0.3/ as I had been able to before. I have the dreaded 'Secure Connection Failed' (Error code: sec_error_reused_issuer_and_serial) error screen.

I had PREVIOUSLY had this problem when I initially installed the printer and followed the very helpful workaround instructions at

https://support.mozilla.org/en-US/kb/Certificate%20contains%20the%20same%20serial%20number%20as%20another%20certificate

but they didn't work this time. I deleted the certificate for dhcppc1 (192.168.0.3) as instructed, but the instructions for adding a new exception are no longer valid.

"Magically" this morning when I tried to get to my printer's built in webserver at 192.168.0.3, Firefox allowed me (presented me with the necessary screens) to add an exception for that address. Problem solved although I don't think it was due to anything I had done...