I only know of two ways to avoid the cert error upon visit:
The first way would be to create an exception in FF for this site. The downside to this is that MITM attacks could be performed on this site outside your purview.
The other option, which you don't seem keen on is adding your own generated and self-signed CA in the CA list. Given that you take care of your personally-generated CA, I do not see too high of a risk for doing this. Please do note that I would not do this if the site you are trying to sign is an RSA server or something of grave security importance.
If the cert error is of extreme annoyance and the two options above are not acceptable, you may consider buying a cheap SSL cert from GoDaddy or your entity of choice. Granted it won't be as secure as a Verisign cert, but it's something from a CA (disliked as they may be) that should be trusted by your browser by default. If you have several sites you need to secure in the same domain, but limited liquid assets, you may want to seek out a wildcard cert. A wildcard cert is a SSL cert that will allow you to sign all sites that are under the same domain with the same certificate. Here's the Wikipedia article for Wildcard Certs: