Forum communautaire Firefox

I currently have Firefox to open at startup showing three different websites. When I open a new window, the same three sites are open. I would prefer all new windows to open with one new tab only. I don't much care whether that new tab displays some sort of blank page or home page or a website I have chosen or whatever.

I can't find a setting or an extension which allows this. Can anyone help?

I have vertical tabs enabled and I'm trying to access the tab overview menu to close duplicate tabs. Unfortunately I can't find this menu anymore. If I disable vertical tabs, I can see the icon in the top right. How do

How do I access the tab overview menu when using vertical tabs?

Thanks