I wonder if anyone might have a hint on how I could create a certain behavior for TST.

Generally, I want the TST sidebar to be full width (~250 px) so I can read all the open tabs at once. But when I have several windows open on my screen, the TST bar can take up too much of the window and reduces usability. I'd like to have the TST sidebar to auto-collapse (so it expands on mouse-over) when the firefox window is less than (say) 500 px wide/50% of the screen width.