I would not worry about this. I'm also on a release channel nd can see the same thing as you.

A bug has been reported to see what the bug fixers say. https://bugzilla.mozilla.org/show_bug.cgi?id=1275168

So how did I get onto a release channel? I know I never opted into it.

Check this: In Thunderbird Help > About Thunderbird It may say this: 'You are currently on the release update channel'

Well it says I'm in the release channel. I never opted in to being in the release channel.

There is a bug on what the Start Page is displaying. That is all. It is not the end of the world. Turn the Start Page off or ignore it until they fix the bug. As for the release channel it means that you are using the normal RELEASE version vs being on a beta test or early release version. The latest release version is V45.1 so if that is what you show your world is still in order. You opted to use free software and free upgrades come with the normal release channel.

How to turn off the start page display: 'Tools' > 'Options' > 'General' or 'Menu icon > 'Options' > 'Options' > 'General' uncheck: 'when thunderbird launches, show start page in the message area' click on OK

Start page is a way to see important issues that might occur. So generally, not a good idea to disable it.

The indication that you were on "Daily" was a website issue that got fixed Wednesday afternoon 2016-05-25