Tools/Account Settings, select an account in the left pane, then Account Actions at the bottom of the left pane, Remove Account. Remove is greyed out if the account is the Default, so set another account as Default by selecting it, then Account Actions/Set as Default, then Remove the other account.

Both are marked as Default, hence Remove is greyed out on both accounts. Actually I tried this before. Sorry , it doesn't work.

Create a new profile from Help/Troubleshooting, click about:profiles, and add the account. The current profile is unaffected, and you can copy data such as address books, local mail, calendars etc. from the old profile too the new one.

Can't find Help/Troubleshooting

Click the 3-bar icon in the upper right corner, Help|Troubleshooting Information. Or, press Alt to show the Menu Bar with Help.

I found it and created a new account marked active but this can only be seen on profiles, The other two accounts are still marked active and a test mail has been sent to both of them but not to the new one as it has not got the address. I take it the new account has to be transferred to the list to the left to the other two accounts before these two can be taken away.

I'm not sure what you mean. Accounts aren't created or deleted by TB. That is done with the mail provider. If you had xyz@telia.com shown in duplicate in the old profile, and now you have xyz@telia.com by itself in the new profile, you can delete the old profile, from about:profiles, if it has nothing you need, and xyz@telia.com in the new profile will continue to work.

The computer had another problem today, so I brought it to the place where I had bought it, so they fixed both problems at the same time. Now it's up and running again. Many thanks for your guidance.