There are many sites that have self-signed ("untrusted" in Firefox) certificates. I then have the option to trust them or not. If I not trust them I can either leave the site or only visit the site fully unencrypted.
I know, there are cases where I should not use the site at all if there's the smallest risk of "being watched" (online banking, shopping, and so on). So, the question is more like: On non-critical websites, e.g. sites I only read (news or blogs for example), is there any reason to not trust a self-signed certificate? In the worst case "I'm watched", which I could be anyways if I used unencrypted HTTP.