When I try to access the admin pages with FF 83.0, I get the above error. What is the reason, or how can I fix the problem?

Thanks in advance.

I did a quick search on this and found a range of explanations but there were several that highlighted recreating the self signed certificate solved the problem. This was also found by people who were running pfsense.

If you have been running IPFire for some time then your IPFire self signed certificate could be quite old and maybe some newer required aspects of certificates were not included. Maybe this latest version of FF is finding something missing that it now requires with a more strict setting being applied by them.

If the above is the cause then you cannot “fix” it, you can only create a new self signed certificate for IPFire.

How to do this is described in the wiki:-