Moving Thunderbird from an old laptop (W10) to a new laptop (W11). Doing this for a non-computer-literate friend. Old laptop (bought in 2016) is now running W10 but I assume it was originally W7 and upgraded at some point (by me). I followed [https://support.mozilla.org/en-US/kb/moving-thunderbird-data-to-a-new-computer Moving Thunderbird data to a new computer] which seemed straightforward. However when I copied the Profile across and restarted Thunderbird there was no data to be seen. I tried again just to be sure, including uninstalling and reinstalling Thunderbird. Still no joy. [There are support threads about this but they seem to be archived so I can't update them.] Onwards and upwards. I tried the "export to zip file" option. This had a mild warning that if the Profile was more than 3GB it might (!) be better to back up instead. [Noting that I had already tried this and failed - see above.] It produced the ZIP file eventually, at an impressive 15.8GB. I then tried to restore from it, but the Restore option refused to find it, showing a comment that the file must be not larger than 2GB. I refer the reader back to the note on the Export option. I then noted that you could Import from another Profile. I had one of those saved from my first attempts to copy Profile -> Profile. So I tried this option and it worked. I think my issue was that the old version of Thunderbird was 32 bit and the new version was 64 bit. In which case the document linked to above could do with an update to warn about moving from 32 bit to 64 bit and include the method that worked. [Subject to more testing by others to confirm the solution works reliably.] The Export dialogue could also do with a much more specific warning, to align with the Import dialogue. This is a brief summary of a very long day because shifting 15.8GB of data around between computers and in and out as well is incredibly tedious! :-( Noting that the ZIP file and the Thunderbird directory tree with the Profile in are both the same size.