But, unfortunately, I had to go back to Thunderbird. With the exception of free software mailing lists, people simply do not know how to use email. Bottom quoting is the default. We can thank Microsoft for that one, I think. I can't understand why anyone thought it was a good idea to attach the previous message to the bottom. Most people just accept this as the way email is and never question how unbelievably stupid it is.
Then you've got all the HTML shit that people put in there. And the fact that many clients are simply broken and don't write the headers correctly which breaks threading. This is often confounded by broken servers which garble the headers a second time.
It's amazing that something as simple as mutt or gnus (emacs) has everything you need: threaded messaging and convenient composition. Inline quoting is surely the only sane way to do quoting. Why would you assume the person you reply to has deleted their own message? My message to you is right there in the same thread in my client. Don't send it back to me. But do make it clear which parts of it you are addressing (inline quoting). You had to laugh when Google reinvented threaded messaging in the 2000s and advertised it as some groundbreaking new thing.
So I gave up. I use a client that can deal with the stupid shit that other clients spit out. I also bottom quote now. Why? Because if you do inline quoting, MS Outhouse web client thinks that everything after the beginning of the quotation is a copy of the previous message and folds the whole thing, meaning your recipients get back what looks like an empty message. Really.