They’re using a Mozilla based browser to send emails (Mac version.) which could be Safari or Firefox.
Given the fact that it’s a Yahoo account short of getting a subpoena you’re not going to identify the return track.
Am I incorrect?
It was probably generated using Mozilla's Thunderbird email app - similar to Microsoft's Outlook. Thunderbird's current version is 2.0x., whereas Firefox is someplace around version 3.5x, FWIW.