I see that the vertical tab expand-on-hover animation takes 400ms (per about:config), but this is annoyingly slow. I find that when I want to click something on the left side of the page, I often overshoot my target and put the mouse cursor in the sidebar, which immediately expands, costing 400ms.
I then have to pull my mouse out of the sidebar and wait for it to fully retract (another 400ms, so almost a full second) before I try to click again, because clicking inside the area where the fully-expanded sidebar used to live will expand the sidebar again, covering the thing I intended to click.
400ms seems awfully slow to me for an animation in general, so I'd suggest something like 200ms instead. Also, it would be even better to introduce a delay before opening or closing the sidebar (e.g. the mouse has to dwell inside/outside the sidebar for at least 200ms before opening/closing it).
Folks who want to change the animation speed can look for sidebar.animation.expand-on-hover.duration-ms in about:config. 🙂 Setting it to 100ms (or even 0ms) makes things feel much better right now with no code change required.
Or, you can make this issue easier to reproduce by setting it to a large value (like 2000ms) and playing around with the sidebar (hover, move your mouse out, move it back into the area it used to occupy and click).
