I have the folder 863ixu1q.default.

That's your profile folder containing all your data. https://support.mozilla.org/en-US/kb/profiles-where-thunderbird-stores-user-data

This folder contains files and folders dating from 2011. I'm guessing that these are not needed and would like to know how to proceed to clean up 863ixu1q.default.

That's exactly right, you're guessing. Usually deleting stuff based on guessing is a silly idea. So unless you know exactly what you're doing, I'd suggest leave those files alone. There may indeed be files not needed anymore, but as long as you don't have any problems I'd not start messing with them.

Thank you for your reply. Since I don't know the reasons for some of the files in 863ixu1q.default, I best leave them alone. However it is bothersome to me that I have 21 files/folders in this file that date from 2014 and earlier, up to 2011. One would think that there would be some tool to clean up these "obsolete" files.