Having just upgraded Tbird, I see there is now an export option.
I clicked on "Open profile folder" and in MyDocuments>Thunderbird>
cache there are 16 folder named 0 to F and each one contains lot more folders named "00" to "FF" but with some missing so not all possible 256 files. Inside some of these folders there seem to be a pair of files with an 8-digit hex name, though most are empty.
In the top level folder (with 16 subfolders) there are four actual files called _CACHE_00N_ where N is 1 to 3 plus another with MAP in the name.
All these folders and subfolders and files seem to be dated July 2017 but the MAP file seems to have been updated on 6/10/22
Have these been left behind by an older version of TBird ?.
I ask because another folder called CACHE2 holds lots of files with long hexadecimal names and all dated today or yesterday.