I have followed the instructions for hand editing msgFilterRules.dat repeatedly, having exited out of Thunderbird first, but when I restart Tbird I see the old state of the file instantly recreated. Very frustrating. I've searched all of C:\Users\Gary\AppData for any msgFilterRules.dat, and the only other one is the one for my main account (named for my server name/domain), and it's not copying that one. It's clearly reinstating a copy of what it was before.

So all the previously-posted advice about how to copy filters, etc., is quite useless for me, if I want to copy from my main account to Local Folders.