look for msgfilterrules.dat in the account folder under imapmail (imap) or mail (POP) folders in old profile. It is a simple text file, so it's easy to review in case anything changed, such as folder names.

Found the file, which in the older profile is in Mail/Feeds. Where ought I to paste a copy of it into the new profile, which does not have a Feeds folder, but does have both IMAPMail and Mail?

Hmm,, I just noticed that the file is 0 KB, which does not bode well. Filters were working when I last used the old default profile.

you might also check the imapfilters folder in old profile.

I saw your reply on my Pixel7 Pro phone, but not here.

I seem to not have an imapfilters folder. Very strange.

I do not know what happened, suddenly a few replies were sent to my GMail account and apparently could not receive replies. Suffice it to say that David was being helpful, but that my filters seem to have disappeared, although they were functional before I finally created a new and not trouble-causing profile.

I can reconstruct the filters, much easier than doing that for my quite large collection found by the Star icon in the Firefox browser, which I did not put where I finally found it after a great deal of anguish over possible loosing that item.