Where your profile is, there lies your data :)

Help > Troubleshooting Information > Profile Folder. From there, each account has a msgFilterRules.dat file, in imapmail/accountxxx and Mail/accountxxx

Nope. Not with the Flatpak install. The file is nowhere to be found. Both the find and locate commands, in Terminal, come up with bupkis. And neither the imapmail nor mail folders exist. Appears that everything is inside an opaque database.

Which leads me to think that doing a Flatpak install was a huge error. Strangely, it is recommended for Linux users.

I'm thinking I'd like to switch out of Flatpak. But I hate to lose all my message filters.

Why don't you follow Wayne's suggestion and start with locating your profile?

I did try his solution. Take a look at my response and the accompanying illos. But I missed one important element:

It turns out that Flatpak has its OWN profile folders for TBird, separate from the profile folders living inside /home/stan/.thunderbird. The Flatpak profile containment folder is at /home/stan/.var/app/org.mozilla.Thunderbird/.thunderbird And the profiles in that folder DO contain msgFilterRules.dat files.

[ seems like it'd be good to somewhere document that profile folder location part of a Flatpak install ]

Help > Troubleshooting Information > Profile Directory, by the way, neither shows, nor opens, any Flatpak install profile folders.

Thus: now solved.

Thx everyone for assistance.