Thanks Jo*mar.

I chose a password to access my Tbird accounts but stupidly did not save it on any other device. With luck my ISP can sort this for me. I have saved my Tbird profile in the usual format (alphabet soup.default) on my external HD and can see all of my old messages there. Once the password problem is solved I should be able to reinstall it as it was before.

The old laptop died because of a fatal HD crash. Fortunately the important stuff was backed up on my external HD so I have been able to put everything back on this one.