What you should've done use Firefox browser FAQ to move the profile, data, login, passwords using the browser on the W10 computer and move them to files or usb or html files and then use the new setup on the W11 computer to import the data back. You don't just copy files from the other W10 and expect them to work. This is the proper way and copying files will result in corrupted files or the Firefox deleting the data since it doesn't match or can't use it correctly. So you need to get the W10 running with the drive and use the correct backup methods for the data so they can be correctly imported into the new setup.

Thanks Mark, I understand that and would have done it had the Win 10 not died. The Win 10 was running on an iMac using the bootcamp utility until the iMac went bang. The bang has damaged the PSU card AND the logic card in the iMac so repair is costly and unwarranted. I have got the HDD out of the iMac but as you say, all I have is a copy of the profile I was using before. I thought I had read on here some time ago that I could install Firefox on the new Win 11 and then replace the new profile with my old one. Maybe I mis-read.

Sorry all…..my big mistake. I have posted this in ‘Firefox’ when I actually meant Thunderbird. I will repost in the correct forum.

Sorry to waste peoples time.