Ive read and followed two articles here - 1 advocating pasting over the 'Thunderbird' file the other saying delete the folder then paste the backup in its place. Both ostensibly active the result when Thunderbird is started. Everything up and running and all folders in place. BUT re start thunderbird and nothing shows save for three red error messages at the bottom: see photo (inputting verbatim in text sent things awry)

Any helpful suggestions? Too many years of emails backed up here to lose

The red errors on grey background usually indicate an incompatible add-on, probably because the TB version on W7 is older than the one on W10. Does the error appear in safe mode? Hold Shift when you launch TB.

http://kb.mozillazine.org/Gray_bar_below_status_bar

Both are the same TB version (68.8.1) Though I concede that the W7 has had a couple of add ons for a while (Lightning, Manually sort folders, Quicktext, Shrunked Image Resizer) all updated. Yes opens in safe mode showing no errors, but the first box on opening will only process the 'continue in safe mode' button (the disable all add ons & Reset tool bars and controls, though can be selected the Make changes and restart button gets highlighted but does not operate if clicked). TB now will only open in safe mode (cannot make a normal start / get the error screen as before)

Run in safe mode and remove all add-ons from Tools/Add-ons. Help/Troubleshooting, click Open Folder, close TB, delete xulstore.json, to 'Reset toolbars and controls'. While you're there, delete global-messages-db.sqlite, the global search index, as it is a common source of migration issues; it will be rebuilt on TB restart.

http://kb.mozillazine.org/Safe_mode

Did that (no xulstore.json listed though... but rebooting again to safe mode it curiously let me Reset toolbars and controls. But even will no addons etc no ordinary start only safe mode

There is a bug report that seems to point to localization errors causing this:

https://bugzilla.mozilla.org/show_bug.cgi?id=1582936

but it looks to me you're using an English version. It might be worth seeing if the problem remains in a new profile: Help/Troubleshooting, about:profiles. If that works, data from the old profile can be copied over.

Sorry though this looks hopeful youve assumed a larger comfort zone than I possess :( Help/Troubleshooting, about:profiles I cant find and I don't understand how to create another profile or how to subsequently copy over data one to the other

Apologies my bad - OK figured how to create new profile (found in troubleshoot). How do I do the copy over?

Is the gray bar with red text absent in the new profile? If it looks correct, you can copy data from the old to the new profile. First, for IMAP accounts, add them to the new profile, and the mail will download from the mail server. To copy POP accounts and local mail, close TB, open the Mail subfolder of the old profile and look for subfolders named after the account, e.g. pop.gmail.com. Copy the mbox files - files with no extension and named after the folder - to Mail/Local Folders of the new profile. Same thing with any mbox files in Mail/Local Folders of the old profile. Copy abook.mab and history.mab from the old to the new, to copy Personal Address Book and Collected Addresses.

http://kb.mozillazine.org/Transferring_data_to_a_new_profile_-_Thunderbird

Yes new profile no red text (I haven't input any mail accounts) Want to copy several POP account data from old profile but Im not sure of the 'mail subfolder' location (or Mail / local folders on the new) I appreciate the generosity of the help I am being extended but Im getting seriously out of my depth here.

Start TB in the new, empty profile, and add one POP account from File/New/Existing Mail Account. Then, Help/Troubleshooting, click Open Folder, close TB. The current profile is shown in File Explorer, with a Mail subfolder, and subfolders of Mail named after the POP account and also Local Folders. Now, go up one level in File Explorer to show the Profiles folder, which will show the new profile folder and the old one. Open the old profile folder, and then the Mail subfolder. Mail shows a subfolder for each POP account. Open an account folder and note the large files with no extension named after the folders, e.g. Inbox. Copy the mbox files and paste them in the Mail/Local Folders location of the new profile, renaming them if necessary. When you start TB in the new profile, the folders should appear under Local Folders in the Folder Pane.

Perfectly explained...just a couple of questions before I carry it out: I have 4 x POP accounts on the old profile do I set up each in turn manually on the new profile as you explained (above refers to add one POP account)? What happens re the Local Folders folder content on the old profile (several years worth of emails in various folders)? or am I misunderstanding your final sentence above - does that refer to these Old Profile Local Folders, or as I read it to the copied 'Inbox' etc files with no extensions? (concerned that would not end up like the old Folder Pane ie 4 POP accounts at the top of 'All Folders' each with their inbox, drafts, sent etc sub folders. Then below 'Local Folders' with its deleted, outbox, then the 30+ Folders (and their subfolders) that I've filed my email archives in. Thanking you for your continuing help.

Yes, add each POP account manually. I only mentioned adding one so the folder structure of the profile would be revealed. Besides the mbox files from POP accounts, you can do the same thing with mbox files in Mail/Local Folders of the old profile, or ones in .sbd (subdirectory) folders.

Now that I've painstakingly imported all the Local Folders/& sub folders, I assume it will be straight forward to delete the first profile (the one that only opens in safe mode)

Also, can any insight be imparted on how to copy across filters from the old profile on my old PC?

Filters are stored in msgFilterRules.dat, and can be copied from the old profile or between accounts. The file is in the account subfolders of Mail and ImapMail in the profile folder.

If you delete a profile, do it from Help/Troubleshooting, about:profiles.

Many thanks