Hello,
I think there should be an option to customize the delay for expanding the sidebar with vertical tabs in Firefox. Most times when I want to change tabs, I move my mouse cursor over the sidebar, and I already know which tab it is and don't need to expand the sidebar, but I am unable to click and move my cursor away without the expanding animation starting. I think for me a 1-1.5s delay would be ideal (because sometimes I need to read tab titles), but I didn't find an option to change this.