It's more like a web address unless you have a Yahoo mail checking extension installed in Chrome or Firefox. Either way, whether connected via a website or an extension, the connections would show up in netstat.
For instance, close all browsers and run the neststat above. Then open Firefox or Chrome and navigate to yahoo.com. Run the neststat again, you'll see probably multiple new connections there.
Also to consider: If you have a flag in Chrome set to "always run in the background", you'll still have those connections show in netstat even after closing Chrome.
I see you mentioned changing all your passwords for your financials. I'd go ahead and change the rest of your passwords as well. Facebook, Twitter, Email, etc.
