seldenb

Whenever I encounter a problem like this I like to report it to someone who might be able to do something about it. It can be risky to get into the habit of allowing invalid certificates. There's always the possibility that you might be giving your browser permission to access a site that's, shall we say, less than reliable. FWIW, it's working fine for me today (23nov19). FF70 opened https://www.getpaint.net with no complaints.

The results of a Web search suggest that this might be a symptom of the certificate's installation not being configured properly. See, for example, https://support.mozilla.org/en-US/questions/1132690 Apparently Firefox and derivatives (like Waterfox Classic) are much stricter about certificates than other browsers are. For example, Comodo's own Chromium based browser (Dragon) doesn't complain. FWIW, I'm running them under Windows 10 1907. P.S. I contacted Comodo about it. For some reason the person investigating was unable to find a registration for the domain getpaint.net although I suspect that's just an oversight on their part.

When I attempt to access Paint.Net's Web site, Firefox v70.0.1 blocks access by default, complaining that it doesn't recognize Comodo as a certificate issuer, of course it should accept Comodo's certificates unless there's been a recent security breach of Comodo that I don't know about. See the attached screengrab. I'll be reporting this to Comodo (I use their Windows Internet Security product) and Mozilla, but it probably would be a good idea if the getpaint.net admins reported the problem, too.