I am transferring this to the Firefox forum as the issue will be about the Firefox install and what is set as a default in Windows and not something about Thunderbird. Thunderbird only passes the link to windows to open it.

I had to mark it as not resolving the problem, but it also may be pointing to the solution.

The default for windows works from every other package except Thunderbird. If I click on a link in anything else, be that a pdf view or a link in a document, then my default Firfox is loaded.

However when I click on a link to a map, then the same thing happens as Thunderbird, so no it is not specific to Thunderbird per se.

It does however beg the question that maybe there are two lots of firefox for default linked to different things.

I will ofc investigate this.

OK Thanks, I found the problem, due to your comment.

After an update install of Firefox, the original version was still hanging around and had not been uninstalled.

This led to the fact that some files were linking correctly to the latest version and some to the previous version.

The profiles had gone to the newer version so were no longer linked to the previous version, which led to the message above.