What are the settings on the old account? See https://support.mozilla.org/en-US/questions/1417298#answer-1588949

Thank you for your reply.

I was able to set up a new account that used the same email address which received all new mail, but has none of my old mail. Obviously I would prefer to just a single account for the same email address, but really can't see what is wrong with the old account. The new account's settings is using a POP Mail Server (port 995), SSL/TLS connection security, normal password authentication; I've ensured the old account is exactly the same but still no result. I'm nearly certain it has something to do with the update, but I can't see what the update could possibly have done that would just stop it working after 9 years with no hassle.

Cheers!

Could you please do the following? With the troubleshooting information, perhaps someone can spot the issue.

1. Open the menu > Help > Troubleshooting Information, then click Copy text to Clipboard.
2. Go to https://pastebin.mozilla.org/, paste the clipboard by right clicking in the large text area, select paste from the menu, change the retention period to expire in 28 days, use the Paste Snippet button to create a page containing your info, then copy the resulting URL (address) of the page created.
3. Open a reply to this post, and paste the URL to your troubleshooting information you just copied.

Thank you for your reply. I was able to set up a new account that used the same email address which received all new mail, but has none of my old mail. Obviously I would prefer to just a single account for the same email address, but really can't see what is wrong with the old account. The new account's settings is using a POP Mail Server (port 995), SSL/TLS connection security, normal password authentication; I've ensured the old account is exactly the same but still no result. I'm nearly certain it has something to do with the update, but I can't see what the update could possibly have done that would just stop it working after 9 years with no hassle. Cheers!

Hotmail/outlook.com accounts use OAuth2 authentication, not 'normal password', as shown in the link in my first reply. Make the change in the old account, restart TB, enter the account password in the OAuth window when prompted.

My apologies sfhowes, I should have clarified: the account I added recently is a hotmail account, but my original account is provided by a different system. I now have three accounts on TB, my original which is not receiving, my hotmail account which is working, and the new version of my original account which receives new mail but doesn't have any of my old emails; I've set all the settings for my original account to replicate the new version of it but still no luck. At this point I can just retain the two versions but would prefer to merge them.

Thank you Matt;

Here's the URL for that:

https://pastebin.mozilla.org/EoDU8Qqu

Help/Troubleshooting Info, Profile Folder, Open Folder, close TB, open the Mail subfolder. You should see a subfolder named pop3.westnet.com.au (if that's the original) and a subfolder named pop.westnet.com.au (if that's the new account). In the pop3.westnet.com.au folder, find the mbox files - the large ones with no extension, named after folders - such as Inbox, Sent etc., and copy them into Mail/Local Folders, renaming if necessary, e.g. Inbox to InboxOLD. There could also be mbox in .sbd subdirectories. Restart TB and find the folders under Local Folders in the Folder Pane. After you've transferred all the mail, the original account can be removed from Account Settings, Account Actions.

Help/Troubleshooting Info, Profile Folder, Open Folder, close TB, open the Mail subfolder. You should see a subfolder named pop3.westnet.com.au (if that's the original) and a subfolder named pop.westnet.com.au (if that's the new account). In the pop3.westnet.com.au folder, find the mbox files - the large ones with no extension, named after folders - such as Inbox, Sent etc., and copy them into Mail/Local Folders, renaming if necessary, e.g. Inbox to InboxOLD. There could also be mbox in .sbd subdirectories. Restart TB and find the folders under Local Folders in the Folder Pane. After you've transferred all the mail, the original account can be removed from Account Settings, Account Actions.

Thank you very much for your assistance!