what are message rules?

Message filtering rules..

do you have the correct account shown in the dialog that opens when you select filters. They are account specific, so whatever is selected in the folder pane will default into the filter dialog, and that might not be where you placed your filters.

Yes. I have 4 mail accounts and there were message filter rules for 2 of them. All have been disappeared. They worked well for a couple of weeks. There are different folders for different filters, the last message that was moved by a filter to a specific folder was received on the 15th Aug. Since then no messages are filtered, all are landing in the main folder. Also, my ForkLift app disappeared from the dock (this happened a this weekend) reinstalled and it works, even the registration came back, but that's a different story, however still strange.

check to see if the files are still in the profile name account folders. In Thunderbird Help > Troubleshooting Information click on 'Open Finder' button

A new window opens showing contents of profile name folder.

If pop account click on 'Mail' folder If imap account click on 'ImapMail' folder

click on mail account name folder. Message Filter Rules should be in a file called: msgFilterRules.dat

Do you see any file(s) like the following: msgFilterRules.dat msgFilterRules.dat.bak or any other variant?