Root Certificate expired for Filezilla but not for Firefox

Browsers start with the "leaf" you sent them and then make up their own "intermediates". They do this to adapt to poorly configured servers and other reasons. To see the actual chain your server sends use a site like this:

It shows you are sending the default long chain.

Let's Encrypt now offers a default (long chain) and alternate (short chain). This website and many others use the default long chain. Here is more info about these choices: