Did you install TB correctly? See 'Common Problems' here:

https://support.mozilla.org/en-US/kb/installing-thunderbird-on-mac

Thank you for your suggestion but I did all that and still have the issue. Everything works just fine but I downloaded Thunderbird specifically to be able to archive all my emails on my hard drive and this particular feature does not seem to work, or I just can't find the file.

Raymond

What file are you looking for? TB stores a folder's messages in a single mbox file, e.g. Inbox, in the profile folder, which can be opened from Help/Troubleshooting, Profile Folder, Show in Finder. The mbox files are in the Mail (POP accounts) or ImapMail (IMAP accounts) subfolders named after the server, e.g. Mail/pop.gmail.com.

I keep having too many emails stored on my ISP's server and when that happens I stop getting emails. So I was told I could download them all to my hard drive using Thunderbird's File-Offline-Download/Sync feature, then I can delete them off my ISP's server. I have gone through the process of downloading these files, and some appear to be downloading somewhere. From what I can determine they are supposed to live in the "Profile" folder within the Thunderbird folder, which is in the Library folder. But there is no Thunderbiird folder in the Library folder. I am not looking for an individual file, I am looking for ALL of the emails.

I don't know what this 'File-Offline-Download/Sync' feature is. If you added the account as a POP account to TB, the mail in the Inbox will download. If you added it as IMAP, the mail will also be downloaded for all folders, including Inbox, by default, unless you unchecked Message Synchronizing in Account Settings for the account.

Did you follow the instructions to open the profile folder?