CAcert is a free (gratis, free of charge) root certificate trusted by many sites and many distributions and programs. There has been talk about inclusion in ubuntu in 2005 in an UbuntuDownUnder BoF. Yet, sadly this has not happened so far.
You can manually install the CAcert root certificate with the following command:
sudo apt-get install ca-certificates
Then you can activate it in firefox with this command:
You can set firefox to accept that the certificate can validate various areas.