SEC_ERROR_UNKNOWN_ISSUER

[encho]

OS: Linux Ubuntu 20.04.6 LTS
XAMPP: 7.4.26-1

I am getting this error - SEC_ERROR_UNKNOWN_ISSUER - from firefox 115.10.0esr and 122.0, when opening my devel XAMPP site on localhost . I examine the certificate and it looks pretty legit.

Reading on the net it appears that some additional certificate files need to be installed in Firefox https://wiki.mozilla.org/Networking/Archive/Necko/Differences in particular please note these:
".. our users are probably more likely to encounter certificate error pages unnecessarily."
and
"Other browsers have more robust certificate chain processing; ours gets confused in some common situations."

Does anybody know how to fix this issue?