I think you need to describe the procedure you're following.

I don't think I understand what "send messages from an URL" means. Perhaps a mailto: link? And is this problem really specific to Firefox? Does it not occur with other browsers?

My message was not very clear, yes I of course meant a mailto link from a web site on Firefox that is sent from Thunderbird.

Mostly I use Firebird and Thunderbird but I know there is no problem with Edge as that sends the mail using Windows Mail. I will have to do some more tests with Chrome and Opera to see whether they cause problems too.

When I found the problem with Firebird/Thunderbird I cleaned out my sent mails and though that might have been a problem but after one or two mails the problem came back.

The problems do not seem to have occurred when creating a mail within Thunderbird.

Thanks, I will do some more testing with the other browsers.

Mike

Hang on. You set up a default email client in your OS which would be used when you do anything that involves an email client. So how can Edge call up a different email client? Or to put it another way, how do you configure Firefox /Opera/Chrome to use a specific client?

Yes, in the Windows 10 defaults, Thunderbird is my default mail agent and as such is called up by the mailto in Firefox, Chrome and Opera. Nevertheless it appears that Edge ignores this and if I want to share an article using Edge then it does use Windows 10 mail. I guess that perhaps the way that share works in Edge is different to a mailto: I don't normally use Edge but I know that it always uses Windows Mail.

At the moment the conditions that brought about the original error messages seem to have disappeared and I was able to send successfully several url via Thunderbird using both Firefox and Opera. Firefox is my default browser and I only used the other browsers to see whether it was browser related. If the problem reappears with Firefox/Thunderbird I will immediately do some more testing from other browsers (at least with Chrome and Opera). Thanks for you help, Mike

I'm pleased for you that it has started working.

<rant>For me, the wayward behaviour of Edge in particular and W10 in general is sufficient reason to abandon Windows altogether. And it's ugly. So much of what you thought you knew about computers has been torn up and thrown away. I'm surprised and disappointed at the way so many people meekly accept what Microsoft imposes on them. </rant>