you are lucky it is only one. Which one?

It would be better to get you the addon that try and get you onto the ESR branch at this time. That should be done in a once a years windows where both have the same version number

It is called LookOut. It would seem unlikely that there is a real incompatibility beyond just checking the version number. I still hope to migrate my emails and contacts from Outlook 2010.

I don't know what an ESR branch is.

winmail.dat is really something Outlook users should only use if they are communicating with other known Outlook users. Often used when on a business intranet network. But for general use they should have disabled it/switched it off. I usually just educate the user to switch it off. The internet is littered with good advice on this. This is a useful link to send to those users/senders. Often they just do not know they are sending it. https://www.thewindowsclub.com/stop-sending-or-receiving-winmail-dat-attachments-outlook

Lookout addon extension info: LookOut (fix version) 6.7 Works with Thunderbird 115.0 - 140.* https://services.addons.thunderbird.net/en-US/thunderbird/addon/lookout-fix-version/

At the above link - If you click on the 'view other versions' text link, there is also this version: You can get it at this link: https://services.addons.thunderbird.net/en-US/thunderbird/addon/lookout-fix-version/versions/

Version 7.0 Released July 28, 2025 Works with Thunderbird 140.0 - 141.* This version has a strict min version of TB 140.0

Looks like they are working on more updates to stay compatible . So I'm not sure what version you are currently using, but it would help to know or whether you tried the version 7 or not ?!

re :I don't know what an ESR branch is. ESR stands for 'Extended Support Release' You can choose that option when getting a download from :

• https://www.thunderbird.net/en-US/thunderbird/all/

Where it says 'Release Channel' you select 'Thunderbird Extended Support Release' from the drop down options. As of 3rd Oct 2025 current download version for this is 140.3.1esr

Process:

1. Exit Thunderbird.
2. Uninstall Thunderbird.
3. Access your profile name folder and delete the file called 'compatibility.ini'
4. Get 'Thunderbird Extended Support Release' download/install from the link above
5. Start Thunderbird.

Thanks for your extensive response Toad-Hall. As I said earlier, I am using Thunderbird v143, but as you said version 7 of Lookout only works with Thunderbird 140.0 - 141.*, so I was hoping to install one of those versions so I could import my Outlook 2010 contacts and emails as a one-off. It seems like it would be a lot of work but I haven't found another way to do it.

Your question asked about installing an earlier version hence that was the information I provided so that the addon Lookout would be compatible.

However, you mention 'I was hoping to install one of those versions so I could import my Outlook 2010 contacts and emails as a one-off. ' You can import from Outlook using the 140.3.0esr version or 143*

Import them into 'Local Folders' mail account.

In Thunderbird Via 'Menu Bar' toolbar Import > 'Import from outlook' > click on 'Continue' and follow instructions.

Via 'Menu app icon' Tools > Import > 'Import from outlook' > click on 'Continue' and follow instructions.

I did try that, but Thunderbird reports "No profile found". I don't what that means, but assume it is because Outlook is installed on a different computer. Is there some set of files I can copy over to make Thunderbird think Outlook is present, or does it actually need to be installed on the same computer for the import to work?

I did try that, but Thunderbird reports "No profile found". I don't what that means, but assume it is because Outlook is installed on a different computer.

Thunderbird imports from Outlook. It must be installed and functioning on the computer you wish to do the import on. So install Thunderbird in whatever computer has Outlook and migrate the Thunderbird profile to the new computer.

I would also suggest you use the ESR version if you are an add-on user. It updates once a year, not monthly ie the ESR version will be 140 for about a year with regular security releases as point releases in the V140 base application so your add-ons will not need a monthly update to work with the bleeding edge release.

https://www.thunderbird.net/en-US/thunderbird/all/

Thanks Matt, I will try the install on the other computer and see if the import works for me. The vital secret sauce is of course how is the migration of the Thunderbird profile to the new computer accomplished?!

That magic sauce is, fortunately, well-documented:

Here is the link for moving a profile in Windows when installed in default location:

https://support.mozilla.org/en-US/kb/moving-thunderbird-data-to-a-new-computer

here is the info in brief steps:

1. ensure thunderbird is NOT running on old machine
2. enter %appdata% in search window on task bar and press Enter key
3. this should open windows file explorer in Appdata\roaming folder
4. highlight the Thunderbird folder and copy to external media
5. ensure thunderbird is installed on second computer and NOT running
6. enter %appdata% in search window on task bar and press Enter key
7. you should be in Appdata\roaming folder , copy&paste the exported Thunderbird folder there. This may prompt to overwrite an existing folder of same name. allow it
8. start thunderbird and all should be there