When downloading tgz artifacts the downloaded file is zipped an additional time. Firefox does not unzip it when downloading it.

This is just about the same as https://issues.jenkins.io/browse/JENKINS-73381 it just affects tgz instead of tar.gz files now.

You can reproduce the issue with the attached job which creates a example.tgz and example.tar.gz artifact. Download both using Firefox. The files will have different sizes.

It worked with 2.462.2 and is broken in 2.462.3

In 2.463 and 2.464 both *.tgz and *.tar.gz were affected.

Starting with 2.466 only *.tgz is affected.

I believe it's due to the mimetype 

tgz=application/x-gtar

in the winstone fix for tar.gz

https://github.com/jenkinsci/winstone/pull/397/files