I'm glad I found this as I am trying to reduce the amount of apps I use on my phone to the maximum I can whilst not sacrificing on anything as I find it helps free my mind up the less apps I have installed. I know it sounds stupid lol.
So a notes tab in Thunderbird would be great. Especially since I know Thunderbird's Android client is considering a possible calendar implementation into the app so it would be like the ultimate three-in-one application whilst still making sense (email, calendar, notes).