It would be great if Thunderbird had better support for Matrix features like Reply, Forward, React and Reply in Thread.
Currently, the GUI for someone reacting to a message is just another message saying "X reacted to Y with Z". It would be better if it showed the emoji reacted to on the message under the sent message. Plus, it would be good if you could react to someone in Thunderbird by right clicking on their message.
Also, if it allowed you to reply, forward, or respond in a thread by right clicking on a message in Thunderbird, that would be great.