Well, for starters, TB doesn't recognize other applications' use of the messages. That is, if you use IMAP, TB automatically makes messages available, not through itself, but via IMAP. If you use POP, TB knows only the machine it is running on and the host server. In that case, you would need to keep messages on server so other machines could download the messages. I am sure others here will have other ideas, but keeping messages in sync across four machines is a daunting task, as each copy of TB stands alone.

My situation is not so complex, so maybe you can help with that.

I run Thunderbird on each of my computers - that's 3 right now. All are running the same version of Windows 10 and the same version of Thunderbird.

I just want to sync my installations of TB (mail and settings) to be the same on each of the computers (as of today, TB102.1.2, 64bit), like Firefox does. On Firefox, I just wait long enough, or initiate SYNC myself, and because I use the same account on each computer Firefox synchronizes to the current settings, etc.

I understood that TB would be setting that major enhancement up. Is that available? Thanks!

Not yet. That feature is being tested internally (far beyond my access), but will be available in the future, but no date announced. All that has been announced is that it is in development.

I solved this problem on my own, but never came back to share it. (I'm sorry about that discourtesy.) Here's what I came up with:

Instead of having folders under the heading of my email account, I set up a series of folders under "local folders". These are my email storage folders, which I've created on my hard drive.

I've assigned the root of my local folders as being /veracrypt7/contacts/Thunderbird email/. All the local folders I create within Thunderbird end up as subfolders within that "Thunderbird email" folder. I created extra "sent", 'trash", etc. folder within local folders. Under /account settings/copies & folders/, I checkmark place a copy in, and select "sent on local folders". I also check "place replies in the folder of the message being replied to". I do the same for archives, drafts, & templates - all selecting local folders.

I use message filters to route incoming messages into the folders I want them stored in, ON LOCAL FOLDERS.

NOTE: The filters frequently don't work, and also, things I mark as "not junk" still end up in the junk folder. So, I still have to do a fair amount of relocating messages manually, moving them down into the local folders. But, the changes I make under the account name section get propagated into the other instances automatically, since those changes are changing things online - and the other instances will read the online account info, and update their records to match.

I'm running 4 instances of Thunderbird, and this method of using local folders automatically syncs my email, since it's not being stored within the given instance. Instead, it's being stored on local folders, on my hard drive. When I create or delete folders in local folders, or change the contents of a local folder, the next use of another instance of Thunderbird discovers those changes, and updates its list of local folders to match. No effort required from me.

I had to come back and edit this, because I realized that I'd failed to include one vital piece of info that makes my method work: I store all my data on an external hard drive, which get used in all 4 of the operating systems I run. (2 Win10, & 2 Linux antiX). The fact that I mount this external hard drive on each & every system makes the local folders I've set up on the external hard drive also available on each of the systems.

I hope you find this helpful...!

.....(I'm trying to delete this reply, but it won't let me.)