There is another user with a similar problem. What was the size of the Thunderbird folder on the old computer, and how did you copy it to the new computer? On a USB drive or external hard drive?

Thanks. It was about 9.5 gb, and I used a USB drive to transfer it.

And the other user was on a Windows machine, so the advice there did not seem applicable to my situation.

The question is whether the USB drive is formatted as FAT32 or some other type. USB drives can be formatted as FAT32 on OS X, which is necessary for some devices, e.g. media players, that only read FAT32.

Were both Macs running the same and latest TB, 68.4.1?

I had to reformat the USB drive from FAT32 to OS Extended (Journaled) in order to copy the folders in the first place.

I don't know what TB is, I'm afraid.

TB = Thunderbird

Was TB closed when you copied the folder onto the drive and onto the new computer? At what point did the size shrink from the original size? If the size didn't change but some folders appeared missing on the new computer, try deleting global-messages-db.sqlite from the profile folder, as this seems to fix some problems of disappearing profiles.

https://support.mozilla.org/en-US/kb/rebuilding-global-database

Oh, duh. Thanks!

TB was, indeed, closed at the time I made the transfer. Here's what's confusing when I open the profile folder: there are THREE folders there. One has the same filename as the profile folder from my old Mac (randomjumbleofcharacters.default), and was last updated this morning at the time I copied it from the old machine, and is the same size it was before. One (randomjumbleofdifferentcharacters.default) was last updated around the time I copied everything to the new machine, I think, and has only one file in it (times.json). And then there is a third folder (randomjumbleofstillothercharacters.default-release) that was last updated minutes ago, the last time I was using the program. It is much smaller (only 33.6 MB, compared to the 9.5 GB of the original).

Both my original folder and this new one have the global-messages-db.sqlite file in them. which should I delete? And/or is there a way to combine the two?

And I'm using 68.3.1 on the old computer (which says that's the most up to date version), and 68.4.1 on this new one.

The profile you want as default is the large one, and you should be able to launch it, if it doesn't launch by default, from Help/Troubleshooting, about:profiles. You can also set it as Default there if it isn't the current default.

Delete the sqlite file from the profile you want as default, only if it still doesn't open correctly.

The TB version mismatch shouldn't matter; it only matters when, e.g., you try to open a profile that was accessed by 68.4.1 with the older 68.3.1.

That did it! Thank you very much!