Filters are defined per account, but they can be copied between accounts by copying the msgFilterRules.dat from one account subfolder to another. Help/Troubleshooting, Profile Folder, Show in Finder, close TB, open the Mail or ImapMail folders and the account subfolders, e.g. ImapMail/imap.gmail.com, to find the dat files.

Search filters can be defined for any set of folders by creating a Saved Search, under File/New/Saved Search.

Thanks. I will try your suggestion. Might have another question if I cannot figure it out.

I am on a Mac. Searched for msgFilterRules.dat and got several from way back in 2012. Not for current emails. I just recently switched from Apple Mail to Thunderbird because Apple Mail had issues with my new hosting service.

The dat files will show the date modified according to when they were last edited, even if the creation date is much earlier. You can confirm this by editing a filter and viewing its date modified. The dat files can also be viewed and edited in a text editor.

http://kb.mozillazine.org/Filters_(Thunderbird)

Changed a filter then used Finder to find all msgFilterRules.dat files. First was in Doucments where I have my backups from my previous use of Thunderbird. I changed to the Mac 2012 which is the latest date on many of the 22 filters. Looked at a couple and they were from way back then. Change a current filter and looked at the msgFilterRules.dat files. None have today's date.

Hoping for an easy way out. Will make one then do the same for the other two or three accounts.

Will search emails for specific criteria and hopefully be able to save that instead of running filters for this project.

Thanks for your help, anyway.

Oh! Just built a filter for one email account after creating a new folder in Local Folders called Civil War. Then went to the next account I was sure I sent to another Civil War author and created the same filter. All messages were filtered properly from both accounts. Lesson learned.