-Do you use any add-on? -Do they work in 38.? -Are they up to date?

Of course I have couple of them: Enigmail, Compact Header, Remove Duplicate Messages, Get all mails, ... and they've worked well.

After your question I tried to restart TB in safe mode, but nothing changed - i.e. no filtering.

I forgot to mention: function "Create filter from message" doesn't work for me too.

In your profile there are msgFilterRules.dat in most account folders (under both Mail and ImapMail)

One can look like this:

version="9"
logging="no"
name="support"
enabled="yes"
type="17"
action="Move to folder"
actionValue="mailbox://nobody@Local%20Folders/1-Support"
condition="AND (from,contains,support.mozilla.org)"

You can send me direct, your troublesome filter (my email in my profile)