These steps work on Windows, but I don't know about Mac:

1. start thunderbird on old PC
2. click tools>export and then click 'open profile folder'
3. exit thunderbird
4. copy profile content to external media
5. install thunderbird on new pc, downloaded from thunderbird.net
6. in receiving PC, click help>troubleshootinginformation
7. scroll to 'profile folder' and click 'open folder'
8. exit thunderbird
9. now, paste contents of exported profile, overlaying contents of profile.
10. if there is a compatibilitie.ini file, delete it
11. restart thunderbird

>Thank you David for your reply yesterday; I don't think it will work on a Mac. Problems seldom come singly and I now find that i cant get >into my wife's Thunderbird account on the new Mac Mini. But on the laptop I have no problems. Both accounts have the same addresses >yet the email address on the new computer will not accept the password. Any thoughts anyone?

You may do well by posting full error message and screenshot of your incoming server settings pane. From your words, I infer that you were able to install the profile? Or not? Please bring us up to date. thanks.

Sorry for the delay. I have been trying to attach a screen shot to this reply without success so far. I will persevere. No I have not installed the profile file yet: I am waiting until I am in control of Thunderbird before I risk it. In the meantime i believe I have discovered the route to the profile file. The method is as follows: From the dock press FINDER, go to GO and hold down the OPTION key. In the Go drop down list LIBRARY will now appear. Click on this. Go down the list to Thunderbird folder and from there to Profiles folder there you should find a file called PROFILE.INI. I am afraid to go any further at this time. I am hoping that when Thunderbird finally lets me in I will find that the profile file has emigrated automatically from the old computer thus making the manual movement unnecessary.

For reasons I can't understand I have not been able to move an image of the error message to this platform. I tried screen shotting it but it would not Airdrop.So I took a photo of it on my phone and this I could transfer but I could not get it to drag and drop onto this reply. So I am reduced to discribing the windows in question. The first one "Email Address: vibeke@redland.plus.com" "User name: Redland" "Password ***********" "Account Type IMAP" "Incoming mail server" "Outgoing Mail Server" "Unable to verify account name or password"

worth saying I don't know the mail servers but it is possible i might be able to find them after another long search

The second window is as follows

"Login to account "vibeke" failed" "Login to account "vibeke" failed" "Login to server mail.plus.net with user name vibeke failed" "Enter new password" "cancel". "Retry"

Just a possibility: change the user name to be the full email address. Many email providers require the full address.

Tried that. No difference I'm afraid.