Message Filters are stored in this file : '''msgFilterRules.dat''' The 'msgFilterRules.dat' file is located in the mail account name folder in your profile. The filter seems to be only available during the session, so I'm wondering if there is a problem writing to this file. Perhaps it has a 'Read only' attribute which needs removing in order to write to the file. * Menu icon > Help > More Troubleshooting Information * Under 'Application Basics' - half way - profile Folders - click on 'Open Folder' A new window opens showing the contents of your profile name folder. * Exit Thunderbird now. If the filter is for a POP account: * click on 'Mail' folder to see contents If the filter is for an imap account : * Click on 'ImapMail' folder to see contents * Click on the mail account name folder for the account. * Look for the 'msgFilterRules.dat' file. * Right click on the 'msgFilterRules.dat' file and select 'Properties' * Under 'Attributes' ...is the 'Read Only' checkbox selected ? * If yes, uncheck that 'Read Only' checkbox. If you want to see the contents of the 'msgFilterRules.dat' file, it can be opened using a text editor program like Notepad. At the top it should say: version="9" logging="yes" Then the filters are listed one after the other. This is what a typical filter may look like: name="TB Forum" enabled="yes" type="1" action="Move to folder" actionValue="mailbox://edited - mail account data/Thunderbird" condition="OR (from,is,notifications@support.mozilla.org) OR (from,is,no-reply@support.mozilla.org) OR (from,is,mailer@lithium.com) OR (from,is,support-notifications@mozilla.com)"