More information, please. What's you operating system? How did you make your profile backup - did you use Thunderbird's Export feature, or did you make it manually? What do you use to "unzip" it - does your OS have a built-in unzipper, or do you use a 3rd-party utility?

I suggest you unzip it to a temporary folder (anywhere, even on your Desktop) so you can examine it to see if it looks right. Then you can move it into its final resting place.

Okay, I've been continuing to try to open the b/u file. Now windows is telling me it can't unzip it, it is not a valid file. I see the file name "Thunderbird.profile.b... today's date, and the size 6,707. Why would it be invalid? Should I try it again?

Lin: It came out of Win 10, using Thunderbird's export. I'm trying to unzip and import it into Win 11 Thunderbird. Partly I only just read how/where to do the unzip, but now it's telling me the file is not valid. I'm in Explorer. If it won't let me unzip it from there, how do I open it and look at it? I just reread what you said about unzipping it anywhere - let me try desktop and see what happens. "Windows cannot open the folder, file is invalid" Nothing remarkable happened while it was being exported from Tbird, should I try to do it again? Thanks, Stacy

I've been told that Thunderbird's Export feature can't handle profiles larger than 2 GB. Is yours that size or larger? It sounds like the zip file is corrupt.

I have never actually used Thunderbird's Export feature, but if I understand how it works, you shouldn't unzip the resulting file - you use the Import feature on the new machine to import the zipped file. Thunderbird will take care of upzipping it and placing it in the proper place. It looks like you can also import directly from an original unzipped profile, if it's accessible.

I prefer to make periodic manual backups my my profile, so I can recover most of my email in things go bad, like a disk crash, or for migrating to another computer. Can you still get to your old setup with the original profile? If so and you'd like to try it manually I'll type up some instructions.

I'm not sure at the moment of the size, I'll check. Yes I'd seen that large files should be handled manually, but I'd assumed that if it were too large, it would throw an error message of some kind at the time. It went fairly quickly until it got to about 50%, then more slowly to 53%, 57%, hung around for a while to 59%, seemed to stay there a while, then all of a sudden was at 99% and finish. It seemed odd but I had backed it up yesterday (thinking I had the latest download but I didn't, so I cleared that one and backed it up again after updating) and it acted the same way both times. My initial reaction was to use import thinking if it knew to zip the b/u, it should know to unzip the import. It did mention that the size was at or near the limit, so perhaps I missed that fact. I've pretty much spent my quota of time at the keyboard for today (very bad neck), but if it won't impose on you too much, I would appreciate your instructions for doing it manually. I'll take another flash drive and try anew tomorrow. My old machine is still in tact and will be so for awhile. It IS an eclipse today, and while the effects of it will last for a few months, perhaps today wasn't such a great day to try this. Thank you Lin, I appreciate it. Stacy

This main thread for this is at https://support.mozilla.org/en-US/questions/1534416 - however, we might be pausing that effort for a brief time.