The certificate is hardcoded in the executable, and Firefox updates with the new certificate were released quite a while ago. If you run into this issue you're

The certificate is hardcoded in the executable, and Firefox updates with the new certificate were released quite a while ago. If you run into this issue you're either running Firefox at least nine(!) releases behind current, or you haven't bothered to install any patch updates for the LTS release in over half a year. Even if they were to release a way to update the certificate in-place, the people who would need it aren't going to bother installing it. They are the same kind of people running Windows Vista in 2025.