In the message filter window, click the button with Subject, select Customize..., add Reply-to as a custom header, and then it will appear under Subject with the other criteria.

http://kb.mozillazine.org/Message_Filters#Custom_headers

Many thanks: I had tried this, but didn't know I had to insert a hyphen between reply and to. However, after successfully creating a 'Reply-to' custom header, pasting in the 'reply to' address from the junk email, and saving the filer, I ran the filter and nothing happened. I checked the filter again, and instead of 'Reply-to contains abcdefg' it shows 'Subject contains abcdefg'. I edited it back to 'Reply-to', but when I then clicked OK to save the filter it reverted to 'Subject'. I tried this several times, and it will not save as 'Reply-to'. However, when I click on it to edit, in the editing mode it correctly shows as 'Reply-to', not 'Subject'. Any ideas?

In the attached picture, I've created a filter with the Reply-to custom header. If I close the window and then re-open it, the settings don't change. So I don't know why it's not saved on your setup. However, a worse problem is that the filter doesn't have any effect when it's run manually. I know this, because when I do a Classic Search (Ctrl+Shift+F) on the folder with the same custom Reply-to header, results are found (confirmed by viewing the source, Ctrl+U).

However, after looking at a few Bugzilla reports, the filter seems to work manually on existing messages, after rebuilding the folder index (right-click folder, Properties, Repair Folder), and should also work for new messages received after the filter was created.

As for why you can't save filter edits, is the prefs.js file in your profile read-only, as it might be if you copied the profile from a backup on an optical disc?

Many thanks for this. I tried again, and this time I was able to create a 'Reply-to' heading that looks like yours and is correctly saved. However, when I run the filter on the folder nothing happens. I have tried both 'contains' and 'begins with', but neither configuration deletes the message, which is the action I requested (other filters using 'from' or 'subject', etc. always do delete the messages). I had already 'repaired' the folder before doing this.

Try with a test that applies a classic search, as in the 2nd picture in my previous reply, that has the same Reply-to condition as the message filter. That way you can confirm if there are messages that should be recognized by the message filter. If you re-run the filter on existing (not new) messages, be sure to first do a folder repair. Perhaps also test with simpler actions like Star or Tag. The option before/after junk classification might also affect a delete action.

The process shouldn't be this complicated, but unfortunately that is the state of things.