I wholeheartedly disagree. The new "To" field is much, much easier to use, and I'd be curious why you think it's the result of 'trade offs made under the mobile constraint'.

It's not a mobile constraint. Nearly every desktop mail client also uses a single To line.

I mentioned mobile because as far as discoverability is concerned, there is an order of magnitude more mobile users than desktop users, so the vast majority of users will have no problem understanding this concept.

And if you've been a desktop user long enough that you havent used a smartphone then you're probably used enough software that have similar patterns (and used email enoguh) that you shouldnt have trouble realizing that single To line means you can add multiple recipients to the same line.

Who said it's a difference made under mobile constraints?

I've literally never used a mail client that didn't do multiple emails on the same line. On Desktop or Mobile.

And it's never been an issue.

I've had issues editing email addresses in clients with multiple mails per line.

As long as everybody is on your address book it's fine, but if they're not, things tend to get awry.

I haven't tried the new Thunderbird, but I am suspicious of this change as well.

"I've had issues editing email addresses in clients with multiple mails per line."

This was my first reaction when I saw the change - I really hope they get this right as it goes to the core functionality of Thunderbird. If email addresses can't be easily and accurately edited in the To field, it will be a real problem for me.

There'll always be someone who has trouble with a solution you can come up with...

I've used every major email platform and every single one manages to endure despite having emails on one line. Even Outlook does one line these days.

How is it any more difficult to edit multiple email addresses in a single box as it is to edit a multi word subject line in a single box?

Clients tend to get confused about where one address ends and the next one starts. I'm sure there are ways to solve this well, but my personal experience makes me skeptical.

Commas?

Yes, if that's how it's implemented... Gmail doesn't. They wrap your email address text in a button thing. Others prefer semicolons, or was it commas, or was it both?

Its just yet another thing I shouldn't have to think about