I can't see how to edit this Q, so I will have to update - but I was mistaken. It is actually breaking all wildcard letsencrypt certificates, not slef signed ones. The self signed ones seem to be OK.

Does it still happen in a new profile? An easy way to test a new profile is to install Developer Edition and see if it happens there or refresh your existing profile. Make sure you stay signed out of your sync account when testing.

I have tried using a private session and doing a cold reboot, but not tried a completely different profile.

This Q did make me wonder though if it was OS dependant, so I have just tried accessing one of the 'broken' URLs via a Linux VM running Firefox 125.0.1 and that worked fine.

I'll try to install a developer edition (not done that before, but I am assuming it is straight forward) and report back when I have done so.

Thanks for the help so far.

I have installed the developer edition version 126.0b3 and that seems to work perfectly.

Hmmm ... I'll go see about creating a new profile too ... moving forward ...

Thank you zeroknight. Your Qs led me to the (somewhat embarrasing) solution.

In case anyone else is looking for something similar, the fix was simple and in the end nothing to do with firefox directly. Once I found that the dev edition worked, I tried creating a new profile and that worked too - so I started looking at the differences. It was then that I noticed that when I started the original profile the embedded VPN plug-in was starting - which was configured to not recognise local addresses.

I can only assume that as the ones that did work were directly referenced by the local IP addresses that the VPN was being by-passed.

So, basically - I right royally screwed up by starting the VPN for something and then not realising that turning it off would not stop it restarting on the opening of any new tabs and didn't notice it was doing so.

Why is it always the simple things that are so difficult to find and fix?

Thank you again zeroknight - you asked me exactly the right Qs (which I should have thought of myself) and pointed me indirectly in the right directions with those tests. Very grateful. All working as it should now.