I think IMAP is much the best solution to sharing mail stores between computers.

I never recommend or advise changing the Local Directory option. Doing so detaches the folder in question from your profile and makes maintenance and management harder. If you do this, please remember that you have done so and mention it if seeking help.

You could go one step further and place your entire profile in the cloud. For this to work, you need to be running the same version of Thunderbird on all devices, and all should be the same OS to avoid pathname difference issues and add-on compatibility problems. (Since I use Linux at home but am obliged to use Windows at work, this avenue is a non-starter for me.)

Thunderbird assumes it has exclusive use of the profile and to this end locks your profile. Changes to the message store may come about from a variety of actions; user, filters, incoming messages, changes at the server being synchronised etc. Locking the profile means that no independent third-party action can disrupt things. If there is any possibility of concurrent multiple access to your message store (i.e. using the account from multiple computers at the same time) then be prepared for corruption and data loss.

Placing a message store on a cloud based service isn't efficient. By default Thunderbird uses mailbox format, where all the messages in a folder are stored contiguously in a single large file. So a comparatively small change, such as the arrival or deletion of a modestly-sized message means the whole file is seen as having changed and in general must be up/downloaded in its entirety. I'm not completely sure, but I imagine that maildir storage might work more effectively in this respect, as each message is stored as a separate file. However, maildir is a new feature in Thunderbird and apparently has some problems, so I cannot recommend it for general use as yet.

Zenos, Thank you very much for this very detailed and understandable response. Now one thing that has always confused me is the difference between IMAP and POP3. I am currently getting and sending all of my emails through a Cox.net Communication POP3/STMP Server and all incoming emails are downloaded from that Server to my Hard drive into the Local Directory. As a result, if I do not have that computer with me, I can not see or search any of my emails. How does IMAP change that and how will this allow me to see the tens of thousands of emails, Directory structure and responses from any other IMAP Thunderbird login.

If I change to IMAP now from POP3 will I be starting over with a Mail Server and if so, I am back to not seeing my past emails and directories. I greatly appreciate any and all assistance, because this inefficiency in my growing business has been a thorn in my side for years now. Thank you, PMK