...or maybe lighter blue to minimise clash with LibreOffice Writer:

?

First probably looks better tbh.

That's a very good BG!
Is this already the new icon from Thunderbird?

It's the 2018 version on Wikipedia: https://en.wikipedia.org/wiki/Mozilla_Thunderbird#/media/File:Thunderbird_Logo,_2018.svg

I note it's Creative Commons by Attribution - as it's the normal icon, I presume it's attributed somewhere in the installation of the app itself?

I would prefer the second lighter BG as the contrast to the logo is higher

And yes you are using the correct icon

Any more comments on Thunderbird or should I PR today?

To me, the last one was very good 👍

Can you make the background of the same color as the background of the Firefox icon?

@eaglersdeveloper consider that firefox and thunderbird are placed directly beside each other on a fresh ubuntu install

And not only because of this. Thunderbird is a product by Mozilla. It recently received a new icon, Proton UI and Quantum engine like Firefox. Need a single style.

xD
I actually meant this as an argument against 2x purple bg :D

Don't fear that mozilla will have a problem with thunderbird having a light blue bg since we place their logo 100% on it. @didrocks already said, that if owners of trademarks have a problem with that, they will reach out to canonical

Both are excellent. The second one works well with dark environment themes. The first one does too but really fits best with the light themes. The Yaru/Saru icons are a welcome addition. I love the uniformity of them and they look great with every desktop environment I have tried. In fact, your work inspired me to attempt creating some of my own to fill in the gaps.