That is the current release, not an error. I can't speak to the website issue. It doesn't appear from a Windows PC, but I think that's intentional

Released January 27, 2025 https://www.thunderbird.net/thunderbird/128.6.1esr/releasenotes/

But, the official Thunderbird download site is still serving up 128.6.0, possibly a technical glitch?

It will update to 128.6.1 after installation

I agree, the website isn't updated. Puzzling that it has been delayed.

Even the link posted by James above, still only downloads the 128.6.0 version. Something, somewhere is screwed up.

I do not know what is the case with Thunderbird at moment, however there has been a small number of times with Firefox where an update was delayed or stopped being served due to issues with the update until a new update was released.

Apparently there was an automation glitch and the web site missed the udate. Nothing more. I guess it will be fixed when the relevant folks return to work on Monday. It fixed in the next day or two I am sure. Until then download it from here if you need that release specifically,or any release as the site holds a full archive of Thunderbird releases.

Not a big deal, but the download page still hasn't been updated. The only reason I noticed is that when any of my installed apps are updated, I always go to their website and download the updated version. That way, if I ever need to do a fresh install of the OS, I'll have the latest versions of all my apps to reinstall as well.