You might find it helpful to know, that because of the W10 corruption I can only log in as a temporary user so I lose all settings if my computer is turned off. When that happens, which is usually by software suppliers updating, I have to go through the Firefox questions again. My data is stored on an external disk, so is not affected. This is only an intermediate computer and I am currently trying to buy a Win 11 one, So I don't want to do anything about the Win 10 problem, until I get the new computer.

Does Edge open, or do you get an error from Windows?

If the Default Apps control panel (in the main system search box, type default apps) isn't available, I don't know how you can set Firefox as the default browser right now. I guess there will need to be a lot of copy/paste of links.

Sorry for the delay. It was because my email service was down.

Thank you, but:-

As I wrote:- 'I haven't used any other browser. Edge was not set up.,' So I don't understand what you mean by .'Does Edge open'. Edge is fired up via the bottom task bar but it is not doing anything because to do something it has to be set up.

As I wrote:- 'When it asked whether I wanted it to be my default browser I answered yes, but then got a blue message saying I should go to settings/default apps'. The blue message doesn't say who it is from, but I have assumed it is from Windows 10.

As I wrote:- 'I can't use Win 10 Settings because it doesn't do anything because it has been corrupted by a power cut.' As you guessed, 'the Default Apps control panel (in the main system search box, type default apps) isn't available,' because the main system search box will not let me type in.

It seems to me this is an error in Firefox, as I shouldn't be getting what appears to be Windows messages. Should I be reporting this elsewhere?

Generally speaking, Windows decides where to send external links, not Firefox. Before Windows made it difficult, setting the default browser simply required a program to make a Registry change. I don't know whether Firefox has stopped trying to do that or whether it was unable to do that.

Unless Thunderbird has a setting to bypass the default browser and send links to Firefox (I'm not aware of such a setting), it sounds like you need to repair Windows. The following article implies that you could accidentally create a setting for which application to use to open HTTP and HTTPS links, which can interfere with them opening, but maybe it's worth a try in your case: Hyperlinks in Messages Not Working.

I read and carried out the suggestions in the article I was capable of doing, but have run out of steam and attitude at, 'Next, try the Standard diagnostics. It's because, it seems to me, a problem with Firefox. Windows is saying, perform a manual action to make a browser the default. That doesn't strike me as something that needs repairing in Windows. That seems quite reasonable, if it was asked to perform the action by software, which it couldn't do because; the request was faulty, or it's changed, so one cannot do it by software, via the browser anymore..

I need to repair Windows anyway by re-installing it. As I'm expecting to get a new Win !! machine shortly, it appears better to leave it for the time being. I can get round it, in the meantime, by Mozzilla Acounts suggestion, of right clicking on the link to copy its location and post it to Firefox manually.

Although I would have expected other people using Thnderbird with Firefox under Win 10 to have this problem also. Otherwise, it suggesdts my particular Firefox is corrupted. So I suppose I could try reinstalling, if I can motivate myself to continue.

Thanks anyway.