Lets start right at the beginning. Thunderbird has one file per folder. Starting with the next version (about a month out) you will have the option to have one file per email.

There is nothing in Thunderbird like a PST. So before you start you need to accept changes in the mind set that goes with mail. Folders are the only delimiter within Thunderbird. You do not get to attach and detach folders or files. They are either in Thunderbird or deleted.

Ok, I think I can work with that. I assume I can copy the contents of email folders elsewhere to back them up, then delete them from Thunderbird. Then, later, I could copy it back, add it back into Thunderbird, move more messages to it, and repeat. I rarely keep more than 6 months of email in my active email, all others are backed up and not 'active'. It's not entirely clear to me how that would work, but I assume I'd have to prevent collisions among folder names is all. As long as I can copy the messages from one folder to another easily, that should work.

Next up will be converting my Outlook archives, 5GB+ in some 15 files, to .eml files that Thunderbird can consume. Are there free methods to do that? I know there are paid methods, but I want to walk the whole process to see if I like the result before I commit to the change permanently.

install Thunderbird on the PC with outlook, the first thing Thunderbird will offer to do is import your outlook stuff. Note it changes nothing in outlook, it duplicates in Thunderbird not repleaces.

Thunderbird cannot import a standalone PST file. Mail import from Outlook works by using the Outlook program to source the emails, not the underlying PST file. If you are switching to Thunderbird and will no longer have a copy of Outlook then it will be necessary to convert all of your existing PST backup files to Thunderbird while you still have access to Outlook on the same computer as Thunderbird. you can search any pst to eml or mbox converter software because thunderbird support eml and mbox file format i google this problem and found a tool for you, you can download this tool from here Outlook Converter http://www.softaken.com/pst-mailbox-converter this tool helps you to create outlook pst to eml and mbox file format, you can use both formats with Thunderbird. Thunderbird is free and secure email solution. i am also using Thunderbird from last 4 years and i will recommend to switch from outlook to Thunderbird