Created attachment 634455 [details]
Demonstrating minimum message height. We could totally get an avatar beside that.
If I have the Chat tab focused, and I vertically squish Thunderbird, I reach a point where the messages in my Twitter stream no longer display the sender's avatar.
I notice, however, that there seems to be a minimum message height (at least for Twitter) - no matter how small I make the window, messages don't get smaller than this height.
I suggest that the avatars should perhaps scale to the minimum message height for the window.