Hi again, @watford 
You wrote:
I've found the following topic that you posted on 28th April 2024, that leads me to believe that you upgraded from Ubuntu MATE 23.10 ("Mantic Minotaur") to Ubuntu MATE 24.04 LTS ("Noble Numbat") by using update-manager -d :
In that message, you wrote the following paragraph regarding Mozilla Thunderbird:
"(...) TBird is now a snap - no option to it otherwise. I need to set-up again all my email and calendar connections anew. A shame. CORRECTION: about 24 mins later TB 'wokeup' in good shape with all the emails and calendar re-established - so my only guess here is that it needed to rebuild the index under the new snap. I had to make a new launcher on the desktop in any case (icon disappeared) (...)"
This makes it even more confusing to me why you are still getting recent changes (as recent as 3 days ago, on 3rd October 2024) in your /home/user/.thunderbird directory tree 
OK. I've found the following web page on Launchpad that says that each of those "2:1snap1-0ubuntu3" thunderbird apt / deb packages is a "(...) transitional dummy package. It can safely be removed.":
"2:1snap1-0ubuntu3 : thunderbird package : Ubuntu"
https://launchpad.net/ubuntu/+source/thunderbird/2:1snap1-0ubuntu3
OK, good That output shows that the "snap" version of Thunderbird got installed.
Unfortunately, I've now also found the following "Open" bug that was reported 3 months ago (and updated 18 days ago) in Mozilla's Bugzilla for "Product: Thunderbird":
"1907184 - profile not migrating to snap if profile dir is too large"
https://bugzilla.mozilla.org/show_bug.cgi?id=1907184
Quite frankly, under these circumstances, I'm afraid that I am at a loss about what is the best course of action I still strongly advise that, whatever you do, please start by BACKING UP both "/home/user/.thunderbird" -AND- "/home/user/snap/thunderbird" directory trees to some "ext4" filesystem in some external media. Then, read carefully @ericmarceau's earlier and excellent reply in this topic, that he posted 14 hours ago - "Memory and disk usage UM 24.04 - #33 by ericmarceau" - and also @Alarik's suggestions. Also, maybe someone else here in the "Ubuntu MATE Community" can give you some additional advice? Good luck!