First you can enable filter log (in message filters). There might be a clue. In your "profile" \ Mail \ "server-name"\ there is a file msgFilterRules.dat you can open it in notepad and post the part that is your troubling filter: Similar to my part:

 name="support"
 enabled="yes"
 type="17"
 action="Move to folder"
 actionValue="mailbox://nobody@Local%20Folders/Support"
 condition="AND (from,contains,support.mozilla.org)" 

Or maybe you see whats wrong and can correct it. Don't save when Tbird is running. Tbird needs to be restarted anyway.

The sequence of events that's unexpected is:

1) I open Thunderbird.

2) It does a mail run on automatic and applies message filter rules on automatic.

3) It give me the error message that it can't find the folder "Freelancer" and it's going to disable the rule.

4) Here's a snippet from the msgFilterRules.dat:

name="Freelancer" enabled="no" type="17" action="Move to folder" actionValue="imap://cchamb2%40gmail.com@imap.googlemail.com/Freelancer" condition="AND (from,contains,freelancer.com)"

5) I go into the msgFilterRules editor and enable the rule, and it shows "Choose folder". which I do. I run the specific filter, and it completes without error message.

I run the enabled message rules on the Inbox, and it now completes without error, but there are about 30 disabled message rules out of 232 total.

This is what the original message rule looks like before any mail run is done (I'm reinstalling my OS and reinstalling Thunderbird, so I have an original copy):

name="Freelancer" enabled="yes" type="17" action="Move to folder" actionValue="imap://cchamb2%40gmail.com@imap.googlemail.com/Freelancer" condition="AND (from,contains,freelancer.com)"

And here's what it looks like after it's been corrected and Thunderbird is closed/reopened:

name="Freelancer" enabled="yes" type="17" action="Move to folder" actionValue="imap://cchamb2%40gmail.com@imap.gmail.com/Freelancer" condition="AND (from,contains,freelancer.com)"

I'm puzzled. Just a thought thou as the folder is on a gmail IMAP-account. What if you marked the folder for offline use? (account settings / synchronization and storage / advanced)

The folders that are affected are currently marked for offline use. I haven't checked every folder (affected or not), but I suspect that being marked for offline use is a default.

As I note that the affected rules, as changed, are showing the folders as gmail rather than googlemail folders, I am receptive to the possibility that it's not a Thunderbird problem, but a Gmail change.

I have the issue resolved. The Mozilla ISP database for some reason is returning the "imap.gmail.com" server name when the account is configured in Thunderbird.

Manually configure it to be "imap.googlemail.com", and the filters work just fine.

Glad to hear