Is there a way to make Firefox ignore invalid ssl-certificates?

Try Add Exception: FireFox -> Tools -> Advanced -> View Certificates -> Servers -> Add Exception.

I ran into this issue when trying to get to one of my companies intranet sites. Here is the solution I used:

1. enter about:config into the firefox address bar and agree to continue.
2. search for the preference named security.ssl.enable_ocsp_stapling.
3. double-click this item to change its value to false.

This will lower your security as you will be able to view sites with invalid certs. Firefox will still prompt you that the cert is invalid and you have the choice to proceed forward, so it was worth the risk for me.

Go to Tools > Options > Advanced "Tab"(?) > Encryption Tab

Click the "Validation" button, and uncheck the checkbox for checking validity

Be advised though that this is pretty unsecure as it leaves you wide open to accept any invalid certificate. I'd only do this if using the browser on an Intranet where the validity of the cert isn't a concern to you, or you aren't concerned in general.

Using a free certificate is a better idea if your developers use Firefox 3. Firefox 3 complains loudly about self-signed certificates, and it is a major annoyance.

Instead of using invalid/outdated SSL certificates, why not use self-signed SSL certificates? Then you can add an exception in Firefox for just that site.