> Cons: It's not the prettiest software in the world, but it's not bad either. the UI is at this point pretty unremarkable, so while it doesn't amaze, it doesn't confuse too much either

You can choose from hundreds of community-contributed add-ons to customize Thunderbird's UI, or you could write one yourself.

https://addons.mozilla.org/en-US/thunderbird/complete-themes...

> Cons: I don't know its keyboard shortcuts, or even if they exist, well yet, so I'm using the mouse a lot

https://support.mozilla.org/en-US/kb/keyboard-shortcuts

Thunderbird at this point is mostly maintained by community members. Feel free to get involved.