Inbox.msf,
.msg FilterRules,
.popstate

''Please continue at https://support.mozilla.org/en-US/questions/1375256'' ''Thank you Stan and David for your replies which were helpful but unfortunately I didn't express myself clearly enough for which I apologise. My USB Thunderbird back-up is from a LENOVO Laptop. The Thunderbird version is 91.2.1 (32 bit) running on Windows 10. I have new DELL Inspiron with Thunderbird 91.8.1 (64 bit) running on Windows 11. I have successfully found in Mail - * Local Folders which contains .msf Filter Rules DAT folders for Trash (File), Trash.msf (MSF File), Unsent Messages (File), Unsent Messages.msf (MSF File) * pop3.iprimus.com.au folders for Drafts, Drafts.msf, Inbox, Inbox.msf, .msg FilterRules, .popstate Sent Sent.msf Trash Trash.msf In the LENOVO back-up Thunderbird there are no matching like named folders or files to input into the DELL. nor are there any storage files with the extensions (mbox) or (sbd) folders. SEPARATELY when I initially setup the new DELL I put the back-up USB is port and Thunderbird initial set-up provided a command to load data from that drive which worked ! In that case I selected the top option IMA ?which stored data on the cloud based server ? not the POP option. What led to this major issue was a need by DELL to delete and repartition the SSD to resolve a problem which I discovered with AVAST and resolved. It was one of these parties that suggested that Thunderbird would automatic convert the old files and set-up appropriately. In all there were 6 individuals involved in at least 2 countries. I have spent countless hours on this new DELL PC with Windows 11. My time is quite stretched hence my delay in responding to your good responses. Hopefully there's some additional info here which will help resolve the problem of retaining all msgs, folders and addresses. Thanks again for your time in considering my problem and providing possible solutions. '' I set up a new Dell 5510 Inspiron with the latest version of Thunderbird which runs well in Windows 11 environment. Previously when I set up a new PC I followed instructions which created a backup from the superceded Windows 10 Laptop on to a USB file Mozilla Thunderbird. I installed Thunderbird and compared the files and discovered 1 file which had no data in it and copied that data directly into that file and it worked perfectly - bringing across everything contacts, folders etc. On the new DELL I followed the setup prompts and had no success - I activated the top box to store data, not on cloud but to retain on server. I uninstalled and tried again this time activating the box under the top one to store most historic data on Laptop - again no success. - comparing data was fruitless as there was no file with 0 data It has been suggested that (with Thunderbird closed, I just copy the Back-up USB Folder "Mozilla Thunderbird" from the Window 10 Laptop over the "Mozilla Thunderbird" in Apps under Settings in Windows 11 and magically all (Contacts and email history in folders ) is supposed to happen. Reason is that the new Thunderbird App does all the conversion and set-up. reluctant to do this as it's taken 2 weeks to reach this point and I don't wish to over-write those Thunderbird that have been received in that time. Your advice would be appreciated. I the process as it has been suggested above OR is there another process to be followed ? Thanking you in anticpation of your time to digest the above and respond accordingly.