If the original profile is still available, I recommend discarding the zip file and using traditional means to copy the profile. Let me know and I will provide details on doing that. If that is not an option, I suggest exporting the zip file so you can copy key components into a new profile. On that: - install thunderbird with a default profile. click help>troubleshootinginformation, scroll down to 'profile folder', click 'open folder' - now exit thunderbird. you are in the profile itself - from the unzipped profile, your addressbook is abook.sqlite and history.sqlite. Copy both of those to the root folder of the new profile - from Mail\Local Folders, copy all to Mail\Local Folders in new profile - IMAP will automatically repopulate so no worry - if POP accounts, now restart thunderbird, set to OFFLINE, and set up the POP account. do NOT retrieve messages. now exit thunderbird and copy the Mail\<POP account> folder to the Mail\<POP account> folder in new profile. Now restart thunderbird.

The problem is that the backup zip file won't open. The account was in IMAP. I changed servers, and before transferring the domain, I had backed up using the "Tools" - Export procedure. It created a zip file that I placed in a folder called "backup." When I then updated the server details and tried to IMPORT, the file was 5 GB, so I tried unzipping it and importing it using "import folder," but then the zip file wouldn't open. I got a message saying "file corrupt." It's 5 GB, so there must be something wrong. I don't know what to do, I'm desperate!

You can download the unzip tool, Winrar, and run in trial basis. It has a feature to fix corrupted zip files. No guarantee, but worth a try.

ok thanks I'll try

I still have the account set up on my phone, can I save the profile from there?

No, and my assumption is that, if the old account is no longer available, that the content is no longer there. Did trying Winrar fail? I did a quick web search and there are several free resources that indicate being able to repair corrupted zip files, so you might pursue one of those.

Unfortunately, WinRAR didn't work. It almost seems like the size (5GB) doesn't contain the data... Can you suggest other programs to try to repair the zip?

I encourage you to search the web for options. I did locate this URL that may work. If it does, please report feedback so I can share with others. https://www.thewindowsclub.com/repair-corrupted-damaged-zip-files

ok thanks I'll try