I want that contacts app to directly access my email client on the smartphone, in this case I'm hoping the Thunderbird app.

There is no Thunderbird app for Android. There are other email apps available for Android though. Pick one which fits your needs.

Wrt contacts, you bought an Android phone, and Android is made by Google. Therefore simply use Google contacts.

For syncing contacts between with Thunderbird, use this extension. Make sure to read the FAQ before you start messing with it. https://addons.mozilla.org/en-US/thunderbird/addon/gcontactsync/

I'm currently paying Yahoo! $20 a year for its pro service,

This is beyond the scope of this forum, but I have no idea why you'd want to do that, and what you gain from paying for Yahoo,

I was going to ask my question via chat,

There is no Thunderbird chat support. This forum is the official Thunderbird support venue.

Thanks, that's very helpful. I pay the $20 per year to get Yahoo! without the ads I guess. Actually, I need to look into that again. I like my email address in Yahoo! If I switch to Google contacts, will I still be able to use my Yahoo! email address?

As for chat, I misinterpreted what it was for. It's simply, you know, 'chat'. But the problem is I was prompted initially to pick among four chat formats and I picked IRC freenode or something like that, and got all balled up. I want to go back and pick Google Talk, but it's not clear how to do that.

You don't get ads in Thunderbird, unless your email provider is unacceptably invasive and adds them to your messages. But $20 a year isn't too outrageous for a personalized domain/email address.

Chat in this context means instant messaging.

You need to add a new account. So File|New|Chat Account in the old fashioned menu. If you use the Application Menu, it'll be hidden under New Message - hover on "New Message", don't click.

You'll probably need to generate an application-specific "App password" in the Google website to use with Google Chat.

Thanks, Zenos. That will take care of the chat issue.

Do you know if it's possible to keep my Yahoo email address while using the Google contacts? Also on my S5, the name 'google contacts app android' brings up a number of contact apps, but nothing that looks like Google specifically.

Do you know if it's possible to keep my Yahoo email address while using the Google contacts?

Yes, it is. But for using Google Contacts you'll need a Google account. This is entirely independent from your existing Yahoo email account.

As said before, since you basically bought a 'Google' phone, you'll possibly need a Google account for other things anyway.

Samsung tend to tweak Android quite heavily. There is usually a built-in app named Contacts which does the job. But the way they expect you to use it is to add or set up a Google account, and in doing so you are invited to select which Google services you wish to synchronize in that account. So your "Google account" links, potentially, to mail, calendar, contacts, Google plus, photos, chrome etc etc.

On this Galaxy tablet, this is under Settings|General|Accounts

Yes, I have a Google email account, but I don't use that address. I don't particularly like the way Google does things, and I don't want to feel stuck in what I regard as their creepy 'Big Brother agenda' web... at least anymore than I have to. I'm getting a book, Galaxy S5 for Dummies. Thank you, Christ1 and Zenos. I'll work my way into Thunderbird eventually and get something acceptable for my Google-flavored Android.

By the way, I'm also going to Linux on my PC rather than stay stuck in Bill Gates magic wonderland and once-a-month-blue-screen-of-death world of Windows. :) Bah bah. You may consider the problem solved, and I'll probably donate to the Thunderbird/Mozilla team.