I was an Outlook user for nearly 20 years. Switch to macOS last year, and along with that went with Thunderbird. It does need some love, but grateful it existed. I like to look at my monthly calendar, open two events, and compare them. Sometimes, I like my monthly calendar visible all by itself, then I'll open emails to the far left or right, then maybe an event in the middle, or to the left.

In other words, I like them all as separate windows, while I compare websites, notes, on the phone, and then adding or editing all these things while talking to people too.

Making the Calendar view, an the list of emails as two separate windows would be amazing.

Thank you Thunderbird team!