The file that stores your filter settings is called "msgFilterRules.dat". Each account will have its own "msgFilterRules.dat" file, stored in the "Local Directory" for the account in your profile. The Local Directory is specified at the bottom of Tools -> Account Settings -> Server Settings, right by the Browse button. The filters for Local Folders are stored in the Mail\Local Folders directory in the profile.

If you want to duplicate an accounts filters in another account you don't need to create them again. Exit Thunderbird and copy the "msgFilterRules.dat" file instead. If the account is in another profile and it uses custom headers you will also need to copy the mailnews.customHeaders setting from prefs.js.

I found the file differently then you indicated. On the thunderbird menu tools there is no account option as you said. However selecting of the mailfolder localfolders you can select accountsettting. There is the directory of the localfolder displayed. Now I also see the otherway. In the menu choose options (not tools) there you can select account settings, then select in that menu local folder and it will display the local directory as in the other method. The rest is as you said, thanks.

Sorry, I copied the folder "msgFilterRules.dat", but nothing changed. In fact when I delete this folder Thunderbird still remembers the filter rules. Thus they must be elsewhere on my computer.....