Hello,
That’s because you’ve already accepted YunoHost certificate before then did a reinstallation (which reset the local CA generated by YunoHost), Firefox consider this to be a security issue (because normally a CA root certificate is never reset or something like that).
To solve this, you need to browser firefox certificate manager and remove the previous certificate you’ve accepted for this domain. To warn you: this certificate manager is … no very intuitive -_-’
It’s this stuff if I’m not mistaking https://support.quovadisglobal.com/kb/a41/how-do-i-check-my-certificates-on-firefox.aspx